Because the libraries keep people on Python and you'd need to support all those too.
Edit to add, my understanding is that the flexible nature of Python makes it hard to swap out the python without implementing all the stuff that makes it flexible. In a way I guess pypy is the python you're talking about. And it does exist, and is faster, but doesn't support all libraries.
Also, "clean" JavaScript can look like CoffeeScript which look very much like Python/Ruby, you can almost say that "clean" versions of languages tend to converge to something like that for some ppl
That's basically Cython. Cython normally gives you 40-100% speedup over python without making any changes to the code and can often give you 10-100x speed up with some relatively minor changes to the code.
That sort of focus is expressed by the library ecosystem, not the language itself. You can write any type of program you want with Julia, you'll just be doing more work if you're building something that can't fully rely on preexisting libraries. As the library ecosystem becomes broader and more mature, this issue goes away.
Edit to add, my understanding is that the flexible nature of Python makes it hard to swap out the python without implementing all the stuff that makes it flexible. In a way I guess pypy is the python you're talking about. And it does exist, and is faster, but doesn't support all libraries.