A LOT of companies change the whole stack just to copy the latest gimmick they’ve seen somewhere else.
Devs need to be constantly relearning how to do basic things, so you either chose some tech and hope it gets popular, or you get to know a lot of frameworks only superficially, which leaves a lot of the internet broken.
SPAs aren't a fad anymore; instead, they're the norm.
Web development over the past 3 years has majorly stabilised compared to the last 10 as a whole.
Want to work on the frontend? React. It's everywhere and can't be considered 'just a fad' in 2021.
Backend? Wrap your head around REST and you'll be fine with most languages (C# with .net core is very common, although often underpaid in my experience).
It might be true, I’m definitely not an expert, I do embedded in C mostly, but I’m always curious to learn the stacks used in each company I get in. Everywhere I go, there’s a completely different one, and all of the leads talk to me about moving to something shinny soon.
My last two places would implement new stuff but in very isolated areas and certainly never "the whole stack".