[netdevnet]

You can't reuse pieces of UI functionality (a bundle of specific html, css and js) unless did copypasta which is obviously awful. Sharing business logic across different areas of an application becomes much harder and you can't non-trivially write and run tests across areas of your application because you are just using html,css and js. And the big monster: state. Sharing state safely across different areas of an application becomes much harder. You end up writing your own micro framework trying to make all the above work. At that point you might as well use as existing battle-tested one

[oneeyedpigeon]

So, instead of "Building websites...", would it be fairer to say "Building some types of web app..."?

[pjmlp]

Sure you can, Web Components exist now, and SSR components exist for 25 years across all main languages.