The issue is that so many man-centuries (man-millennia?) are invested into TeX & LaTeX that sinking time into something else is very, very expensive (much like trying to build a better CPU than amd64 or arm64).

TeX is really, really amazingly powerful. It can do almost anything a typesetter could want to do, fairly easily, and it can do just about everything, one way or another. And its output is heart-achingly beautiful. Sadly, the code necessary to achieve that output ranges from…heart-achingly beautiful to heart-breakingly ugly.

There are other projects out there, of course. I do think that TeX & LaTeX are close to a local maximum, if not al the way there.

XML, in comparison, is a booger joke.