Because they don't understand it and most companies implement it wrong.
I had to read up on it and watch a lot of talks to stop disliking it. The ideas behind the agile-manifesto are great. Ownership to the devs, working together, small iterations, ....
But than consultants came in :) and all of the sudden there is a market and not principles and ideas, but structure and 1 solution to rule them all.
The idea of small iterations gets raped to micro management, the scrum master is not for the team but a PM in disguise or a power hungry colleague; who doesn't understand that he should be there to help, not to control, you.
..... It's still a lot better than waterfall but it doesn't feel like it. Change is always long an complicated.
That's for scrum and lets be honest .... scrum is the only one that made it into big business. So it's the only thing that will be hated .... Bad implementation, egos and so on .... that's my experience with it.
The problem is .... as always the move from "idea/believe" to "religion" an idea/believe is allowed to change and to adapt. Religion is an dogmatic institution that has no room for change because it has a power structure.
If the devs were allowed to actually just pick what they need for their team instead of "doing it the right way", whatever that means. The whole "agile" BS would be nice.