Other companies juggle their priorities differently, and would probably not want to focus their efforts on replacing React, which has a small API, constantly-updated documentation, a team that responds to issues, and widespread attention and thus lots of discourse. I also think the React layer abstracts away complexity, trading total lines of code for cognitive easiness. Some teams have the business justification for doing things their way -- but I think most don't.
(And I don't buy the large teams can't adopt a self-grown code in the first place. If they're any good, they can. If they're not, then even their React and Angular skills will be bad).
Besides you know all your business logic code? That will be custom too, anyway,and they'll have to adopt it...
Average size for the most popular sites has actually grown to ~ 2.5-3MB.
And since they're the most popular, it shows that people DO wait just fine for them to load.
Also what I left out in my argument is that while popular already, they still might get increased in traffic if they were lighter.