Either I've never used Python libraries that would need external libraries, or PIP installs those as well. I definitely know that it can compile code as well, e.g. for installing Numpy/Pandas.
It does not. To keep to the scientific python examples you mentioned: It is a pain to install `matplotlib` with pip if you do not have the C libraries for image compression. (Still, I like pip+virtualenv quite a bit, but they are definitely insufficient when C modules are involved).