Something can be "not about UI" and "the most successful UI platform in the world" at the same time.

Many such cases. :)

Point is, HTML is a document format, not a UI toolkit. That's how it was designed and that's how it continues to evolve.

This is probably not a ideal situation, but trying to change it now probably won't do any good.

Well like it or not but the rise of REST APIs suggests that HTTP is extraordinarily well suited to use as a general interface so it's not too surprising that the related markup language HTML has found similar use as a user interface.