If only world would be so ideal you just described. What if I tell you that back-end starts doing his job blindly, front-end has nothing to do (well, he/they can learn another framework, may be that's the reason we have a lot of them?), design agency sent you only 1st desctop version of index page without any instructions, UI kits and without an information that Grid layout spec will be available in 5 years may be but not now. Next month you receive new information, something changed and you have no idea how to structure your frontend. One more month later your $primary-color becomes secondary or even worse. Later you see in design there is something you have never seen before and didn't evaluated it, in some cases you have no idea how to do that.
Finally after a year you received some parts of UI kit and instructions, you can now see a big picture, but, when you think that you know need to refactor whole architecture, you just close your laptop, smile and leave office, mute your phone, take a random bus into unknown direction and just enjoying your life.