Because pip installs not only the package, but metadata that indicates the package was installed, and where. Without that metadata, the import module_name lines at the top of the python code files would be unable to find where those modules were installed. That's why it fails. That's why doing this in a virtualenv would work, because it packages the metadata with the modules. But that's also why virtualenv is hard, because it has to be in the same path on both the source and the target machine to work.