Why are these "experts" telling so many lies?
Ignorance -- again, not meant as an insult -- is the only explanation that implies no malice on their part. They just don't know because they too never learned it or were never exposed to anything that defined it, taught it, or promoted it. Any other explanation becomes willful deceit on their part, and why it's so frustrating to watch those who actively prey upon said lack of knowledge.
... and there are predators out there, as there is with any other industry. Those who have an emotional or even financial investment in promoting these broken systems. That's where understanding the 7 propaganda techniques and recognizing their use often puts to lie their claims.
Said techniques -- bandwagon, transfer, glittering generalities, card stacking, plain folks, testimonial, and name calling -- exploit flaws in how the mind works.
The human psyche has some pretty severe flaws, and one of the biggest is that when you don't know something, the FIRST answer that gets you through the day is often treated as a inviolate truth -- because "but it worked"... Sure it worked, for you, today, within the limited scope of your understanding at the time. That doesn't make it the best answer, the only answer, or even the right answer! But as part of the brain basically trying to save a few watts, it takes the shortcut of that first answer being the immediate go-to and being resistant to accepting any other answers.
That's what confirmation bias is, and how it leads to an outright cognitive dissonance. When information that contradicts that bias is presented, it is either rejected or worse, twisted into fitting the preconceived narrative. Again why I compare it to cultism, since that's the same mechanism that lets doomsday cults thrive even when the date of the apocalypse set by their prophet goes by with nothing happening.
Hence another of the dangers of learning frameworks first. On top of not knowing enough about what's underneath to actually know if any of the claims about them are true, it establishes that initial bias... and it's why when you DARE to call any of it into question, you get an endless slew of anecdotes, testimonials over how much it helped THEM, dismissive attitudes towards rational arguments, and nit-picking over how it's being said instead of admitting that it's the message that's upsetting them, not the language.
... with not one single fact or example to back up their claims, a dismissive attitude over facts and examples you show, reducing their entire counterpoint to nothing more than "wah wah, is not". Hence why it often feels like talking to creationists, who use one moldy old book written by goat herders who knew nothing of science as their 'proof' whilst ignoring thousands of years of learning, discovery, and rational thought.
Which is part of what's been so frustrating the past decade plus of this nonsense, and why I do often go "off the rails" with the language about it. ... and guys, you were right, that wasn't helping my cause. Just gives the detractors ammunition.
The bandwagon aspect is the biggest part, and not just the "millions of people use it successfully" shtick. Echo-chamber is part of bandwagon that leverages repetition and stamping out of dissent to create the illusion of there being no objections.
It is to that end those of us who advocate the vanilla techniques simply aren't found in the communities discussing this stuff. ANY act of dissension regardless of how politely worded it attacked, derided, and leads to near instantaneous flame wars and bans -- most always one-sided in who gets banned regardless of how polite or impolite the language used.
... hence why those who've spent a decade dealing with this generally dive straight for the insulting language; it's a bit like Picard being pissed off at Q's judgemental treatment of humanity. If we're doing to be damned, let's be damned for who we really are. -- so if we're going to be treated like we're using four letter words for simply voicing concerns, issues, or pointing out flaws, we might as well just go ahead and use the four letter words.
It's a vicious self-feeding cycle.
In that way, places like W3Schools, SitePoint, and so forth end up killing off competent coders so long as they can keep selling their hokey certifications or twenty year out of date books. They foster communities filled with little more than suck-ups and sycophants, all because for them it's more about maintaining their status quo than it is getting people to build websites in a better way... or even properly for that matter.
Which is why such websites are filled to the brim with web rot, bad advice, outright disinformation, and hosts of other failings; and those running them show zero interest in changing that. They want people who will blindly agree because "status quo for the win".
Just look at how many people come pouring out of the woodwork in defense of W3Schools if you dare to say anything negative about it. The site is filled to the brim with disinformation, inaccuracies, and glosses over essential details... and that's now that they fixed many of their older issues. Just a few years ago it was so bad that an many in the industry banded together to create a site called "w3Fools" mocking it.
Because a bad education is as worthless as none.
... and I can prove that the people who create their content are unqualified to tell others how to build websites; just look at their framework: w3.css. For all my complaints about how broken, sloppy, and nonsensical bootstrap and the claims about it are, by comparison w3.css makes BS look like it was created by a genius with deity level understanding of DTML and CSS.
As I often say if you don't know what's wrong with classes like "w3-red", "w3-border", "w3-large" then you haven't learned enough about HTML or CSS to be using either. Don't even get me started about how they reset all font sizes into pixels, an instant WCAG violation.
A situation only exacerbated by their basically having spent the first decade and a half of their site pretending they had something to do with the W3C... they didn't. They never actually claimed it per-se, but the choice of name and their utter unwillingness to correct anyone on the misconception was disingenuous, dishonest, and more than a little sleazy. It took a decade and a half of complaints just to get them to add a disclaimer about that to their site.
To this day, most 'experts' out there will point you at them as the go-to to learn, when what they teach is -- to be brutally frank -- incomplete, inaccurate, or just plain wrong.
... and sadly as the place many people start, many people copy from, and many people mimic,
But more than any of the above, there's just the status quo mentality in action. All of these broken, bloated techniques I'm always ragging on are rooted in two things.
1) the simple fact that people are visually oriented, and expect to be able to say in their HTML what things look like. Again, not HTML's job.. They want to start with the appearance, they only think in terms of appearance, and disregard any other factor.
2) Presentational markup was the norm during the browser wars, and many people have never left that mid to late 1990's mentality. It's all just people blindly copying what others have done without questioning any of it -- even when the governing body of the specifications told us to cut that out twenty years ago.
Both lead to the illusion that you have built a working website, but that runs only skin deep, whilst the ugly goes straight to the bone.
You take an illusionary result, the illusion of ease by preying on ignorance, wrap it in Goebbel's "big lie" by endlessly repeating the glittering generality of "how easy it is", card stacking bad examples as alleged proof?
The masses yum up like it was chocolate soft serve with jimmies every single time, even when what's handed to them is a waffle cone filled with diarrhea and mouse droppings sprinkled atop. Two developers, one cup.
Masters of these methods can convince people to believe in almost anything, and once it becomes belief, it's near impossible to change their minds as that confirmation bias eliminates the capability for critical thought. That's why these methods are the core promotional techniques of every faith, political group, marketer, and are the lions share of the "con man bible".
... and once you start to recognize the techniques, see through it, and the outright lies being peddled? It's like suddenly seeing the world through Hoffman lenses for the first time.
Also why I say "show me something that changes my mind. A fact, an example, anything?!?" -- and in not one single case has anyone even tried. Instead they say I'm being elitist, naive, idealistic, unrealistic, or simply -- again -- cry "wah wah, is not"."
I'm willing to change my mind when presented with facts, true believers rarely are. The trick being to catch people before that cognitive dissonance sinks in, and to convince the handful who retained their ability to think for themselves.
So to answer your question? They lie because they can get away with it, people want to believe it, and it lets them control the narrative. The same reasons preachers, politicians, and con-men like through their teeth.