A big problem is also support by libraries. Back in the day, you could find dozens of libraries to do what you wanted and they worked regardless of your stack because it was all JS anyways (the exception were jQuery plugins, but jQuery is pretty good about not fighting native APIs, so you could still integrate those even if you weren't using jQuery elsewhere). These days, it's pretty hard to find modern libraries that just give you a CDN URL and an init function - most don't even build shippable sources because they expect you to use webpack or whatever, not to mention, that you can find the exact same component ported to Vue, React, Angular etc. but very rarely also to "native DOM", for lack of a better term.
- Page navigation becomes easier if you know the server can handle any route you throw at it as a launch point.
- You can get by with cheaper end-to-end tests for critical flows by not using Javascript in your test suite (although you should be careful to use it when it's important)
- You get a degree of protection if something does go wrong on the front end that you get a non-JS experience to fall back to.
- Pages that genuinely don't need Javascript (think like simple CRUD forms) don't have to pay the SPA tax.
Setting priorities is hard and developers want to try new shiny stuff for fun and also to feel up-to-date.
It usually goes like this, some developers pitch framework X, promising to improve metric Y. In the process many parts of the system change, making it incredibly hard to validate the improvement on metric Y. There's also lack of incentives to validate tech choices. On the other hand being familiar with many frameworks makes some devs feel more senior compared to their colleagues or competition.
Backend development suffers from this as well, you'll be surprised to learn how many nano-scale companies, employing a handful of developers practice buzzword-driven development, with a stack consisting of microservices, Kafka, NoSQL, lambdas.
My own project is in C++ :) Not that it makes anything easier, but I'm not OP, just a full-stack web dev, so ... :P