What would I want to do with Python in a web browser? I wouldn't be able to use most of the standard library, and I wouldn't be able to use most PyPi packages.

Sure, it might be a better language than JavaScript, but you would have to rip out and replace the entire standard library, and Python without its libraries wouldn't be any fun.

Oh we wish. People tried (brython came close). But any attempt are currently not practical as the runtime is very heavy, not to mention the stdlib.

So we are stuck unless the browser vendors decide to ship a Python VM next to their JS VM. And they won't do that.