I don't understand the fuss about SSR, wasn't that default way of doing websites 10+ years ago, before AJAX? E.g. Apache Struts has all of that Router logic since like 15 years ago, completely server-side.
But if one wishes, JS can be added, of course (e.g. for DHTML stuff). To avoid render blocking there are "defer" and "async" attributes.
To answer your original question in a pragmatic way: people used to JS tooling, libraries, frameworks obviously want to continue using these, not revert to Struts. Plus, modern tooling facilitates mixing SSR and CSR, and switching between them as needed.
Finally, you'll end up using React for easy to maintain. And you'll have to wait to load React + related packages => it is slow for initial render => and SSR solved the problem.
Also, you just need a single team to build a complex application while using React instead of backend + frontend as traditionally.