MathML was created specifically because TeX is not a standard, has no definition, has only one “true” implementation and does not capture enough structure of an equation to be a useful interchange format. Even ignoring syntax, the AST of TeX does not represent the underlying equation particularly well - just enough to use to as input to a typesetting system which behaves exactly the way that TeX does, e.g \sum has no way to specify what is being summed over (it’s irrelevant for typesetting in TeX - though not in general).

Math typesetting is hard and stretchy characters in particular are not an easy fit into the browser layout model. It’s not been a priority for browser vendors. Hopefully that will change.