I'll give my guess - it's because of rhe "fullstack" bullshit.

I am a backend developer. I like being a backend developer. I can of course do more than that, and I do. Monitoring, testing, analysis, metrics, etc.

I can do frontend development, of course I can. But I don't like it, I don't approach it with any measure of care. It's something I "unfortunately have to do because someone who is not a developer thought that declaring everyone should be doing everything was a good idea".

I don't know how to do things properly on the front end, but I definitely can hammer them down to a shape that more or less looks like it should. For me, shit like Bootstrap or Tailwind or whatever is nice. I have to spend less time fiddling with something I think is a waste of my time.

I love working with people that are proper front end developers for that reason, and I always imagined they would prefer things more native such as plain CSS.