Because it's not really a dependency manager, just a "dumb" installer and uninstaller. It's for this same reason you can specify an incompatible set of dependencies in requirements.txt and pip won't bat an eye.
Is there any "dependency manager" on top of it that people can use?
Poetry is pretty decent.
Only up through pip 9; 10 and beyond refuse to install incompatible dependencies, and is why we're stuck on 9 for the moment.