Because front end specialists only think about the front end.
The tide is slowly turning though with Next, SvelteKit, Remix, etc. But still these frameworks are basically just rendering HTML and going full on fat client which is overkill for 80% of interactivity in web app but adds a lot of unnecessary complexity.
As a primarily frontend-developer, I can attest to this sentiment. I should add that most backend devs I know understand the basics of HTML, CSS and Javascript, but they're less interested in learning new framework/library/template system constantly. I have to admit, I share that attitude.
It's funny how webcomponents alone are touted as an "alternative" to a framework, despite only being one component (no pun intended) of something that solves the problem of UI state-synchronization on the browser. A better comparison would be comparing a combination of custom elements, server-side rendered HTML, and proxy objects to some SSR framework like NextJS, Remix, etc.