Pip is a huge and convoluted program with tons of bugs. It does a lot more than just download Python packages and unpack them into their destination. Obviously, if you want something simple, then HTTP client, which constitutes only a tiny fraction of pip would be a simpler solution, wouldn't it?

In practice, pip may not honor your requirements.txt the way you think it would. Even if you require exact versions of packages (which is something you shouldn't do for programs / libraries). This is because pip will install something first, with its dependencies, and then move to the next item, and then this item may or may not match what was already installed.

The reason you don't run into situations like this one often enough to be upset is because a lot of Python projects don't survive for very long. They become broken beyond repair after few years of no maintenance. Where by maintenance I mean constant chasing of the most recent set of dependencies. Once you try to install and older project using pip and requirements.txt, it's going to explode...