Frontend frameworks solve a need for SPA (Single Page Applications). While simple HTML/CSS/JS can handle simple static sites, something that needs reactive components (e.g. interacting with something on the site, and populating change instantly) is much easier in a framework. Javascript natively is quite difficult to do this and requires a lot of code to handle simple logic. This is what libraries are good for. React and Vue are such frameworks. These frameworks also allow for iOS and android integration as well.

However, I do agree with things being overcomplicated. I keep my tool chain process as lean as possible, with as few dependecies needed. I find it baffling that people would use React for something like an ecommerce site. Its all about the right tool for the right project. Not everything benefits from React, but some things do some things do not.