I think it's mostly due to nobody solving "it" yet. If you look at the various frameworks, they approach the same problems in wildly different ways, even if the underlying ideas are similar.

Take Svelte vs. Angular vs NextJS for example - they all sort of do same thing (create web apps), but take very different approaches. People choose these various frameworks for reasons, including what style makes the most sense to them, which one is easier to use, which one might be more performant for their use, etc.

There's no clear "right" answer to which framework is best - in other words... the field has not yet been min-maxed.

Regarding server-side rendering - I'd assert today's SSR is not really the same thing as classical SSR. Today's SSR is almost always a hybrid with some sort of client-side payload backed by server-side stuff, giving you the best of both worlds.