You absolutely can, but it's way more common to only know how to create a blank site with a framework than to migrate an existing site. React used to have a section in its documentation for how to add it to your site without adding a build system (not common knowledge, unless it's setting up a blank site through a script) and they've since replaced it with a brief tutorial telling you to migrate your website to a JS module with a link to a suggested build system.

All of this matters because the person that has to do this setup might not be you. It's certainly a problem with the frameworks themselves that porting a website (even just to add a little functionality) isn't as easy as setting one up initially but there's not really much an individual can do here. It's one reason I like frameworks like Preact so much, they're actually trying to fix this.

No because a page-load e.g. breaks the animations.

I mean let's not go too far and pretend there is 0 advantage to JS