We have web assembly and the power to run whatever you can manage to compile with that. There's no real need for "native" support.

The key issue is that the latex stack wasn't really designed to be packaged up like this. It just has a lot of moving parts that are vaguely dependent on running in a full blown unix like shell environment. So the resulting code would be a rather big blob. Running that in a browser isn't that hard if you can live with that having a fair bit of overhead. This has been done. But it's a bit overkill for publishing content on the web.

Browsers don't have native support for MathML any more for a good reason. Mozilla did support this for a while but dropped it because of limited adoption and high maintenance burden. Rendering formulas is a bit of a niche problem and the intended audience is just kind of picky when it comes to technology and generally not that into doing more advanced things with web browsers. Also, most people writing scientific articles would be writing those for publication and probably use Latex any way. So translating all their formulas to MathML is an extra step that they don't need or want.

At least that's my analysis of this. I'm not really part of the target audience here and I'm sure there are plenty of MathML fans who disagree with this.

In any case Katex makes an acceptable (to some) compromise by packaging this stuff up in a form where it can be run server side and is easy to integrate on a simple web page. A proper solution with buy-in from the scientific community (for e.g. MathML) is a much bigger/harder thing to solve.

IMHO, a light weight solution based on web assembly could be the way to go. But of course the devil is in the details because if the requirements are "do whatever latex does" it gets quite hard. And anything else might be too limited.