But many are! This is the issue. 10-50x the effort, complexity, hours, expense. Lots and lots of steel, everywhere, because managers heard that Megacorp uses steel and now insist on it.
Steel beams everywhere.
I don't use React because I'm being forced to. We chose React!
For websites: we all like to dump on PHP but it was literally designed to build websites with relatively limited features / large reach - which covers a large swathe of the web.
So SPA: good for ERPs, CRMs etc but bad for webshops, social networks, TODO apps, news sites, Streaming sites, blogs etc.
[Why Angular? React is technically interesting, especially that it's build around the Rx pattern popularized in dotnet, however I have a deep-seated hate for Facebook for their key part in making Brexit happen which trumps any technical merit. Vue wasn't a thing back then]
Most websites shouldn't be built with React. Most single-story buildings shouldn't be built out of steel I beams. That doesn't make them a bad material, it just means you shouldn't use them if a simpler material will do.