This is a bit like asking "Why doesn't every motor come with a car around it?"
While a car without a motor is pretty useless, the inverse doesn't hold: Most motors aren't even used for cars. Of course one could try selling a complete car to people who want to have a generator, but they wouldn't buy it.
Also the people designing cars might not be the best to build a motor and vice versa.
Similarly with python. Most python distributions are not used with numpy, scipy or pandas. Distributing python with those packages would create a massive overhead.
However, there is of course a strong demand for prebuilt distributions which combine those modules with a respective python and make sure everything interacts smoothly. Some examples are Anaconda, Canopy, python(x,y), winpython, etc. So an end user who simply wants a car that runs, best chooses one of those, instead of installing everything from scratch. Other users who do want to always have the newest version of everything might choose to tinker them together themselves.