Plus the guy gets paid well. Why should he care about React?

You don't generally want (and really shouldn't) to be on the leading edge in a corporate setting.

The reason they don't is that is simply isn't needed in their work.

There is your answer. Most people want to have a job where if they are paid well and are reasonably safe, they don't care. I am not saying its a good or bad thing. It's just the way it is.

You cannot convince someone to pick up new things because YOU think thats what they should do. I personally think people should always be learning new things but I cannot expect my friend to do it if thats not what they think.

Contentious opinion: most 'full stack' developers are bad at least one half of the stack (and very often both).

Switching technologies for an existing large project is a huge risky endeavor, so sticking with your tech stack is usually the right move.

Conversely, the people who use(d) rust, clojure, zig, etc., those are the "exciting new language" people who are going to drop those and move onto whatever the _next_ exciting new language is.

I know Java. I haven't learned React properly either, but I'm deep into the AI stuff. Kotlin was the high risk thing back before Google adopted it. Now I've had more years on Kotlin than Java.