Well yes, for small simple static projects vanilla HTML/CSS/JS(with jquery though) is probably simpler. The problem is when you reach "real life sized" projects. When a project should do more than show a form and a few images and some text.

Frameworks are useful to standardize how many developers in a single project develop together. It also eases binding dynamic views to datasources (databases or API:s or similar). And as a third i'd say that many of the everyday "gotchas" are sorted out in mature frameworks so the developers can focus on the stuff that actually matters.

I am - on the other hand - one of those who argue that frameworks also bring a lot of evil into development. Especially in JS where frameworks are built on top of frameworks which are built on top of frameworks. But I wont bring out those arguments here since the question was "Why to use frameworks" not "Why to avoid them".

Happy Hacking!