Yeah but TeX has problems on its own which makes it a poor choice as a native language. E.g. if you wanted to create an anchor inside your expression, or apply some DOM manipulations. Then there is the whole problem of TeX not having a clear specification, and being turing complete. Both issues make it a noop for browsers to implement as a markup language. Like if you wanted to dynamically change the color of the denominator in the expression 1/x. Which node would you target, and which attribute would you change, and to what, in your script if the expression markup looks like this:
\frac{1}{x}
<mfrac>
<mn>1</mn>
<mi>x</mi>
</mfrac>
> * Faster user experience, as the MathML code can be served directly and rendered by the browser (as opposed to parsed and rendered with the javascript engine).
Those two wouldn’t be there _if_, as the OP suggested, browsers would have supported a TeX-like way to render math instead of MathML.