It's rare, but I've run into packages that don't install easily with pip, due to not having binaries for a specific Python version + platform (something like this one where the combinations of Python+platform+MacOS version are limited, for example - https://pypi.org/project/virgil-crypto-lib/#files ).

Also there are probably tons of "packages" which were never published officially to pypi but exist on github, that people have made public but decided not to maintain.