Which virtualenv's, regrettably, depend on OS-level packages. so when you go to deploy your app to production, you find you installed some system dependency long ago and promptly forgot about it. The real world is messy.
If this is a frequent challenge, it sounds like there's some technical debt to pay down elsewhere in your testing/deployment practice because it really doesn't need to be that hard.
`psycopg2` is a pip package that won't install without system dependencies. Yes, I use Ansible and the system dependencies are managed. But, out of the box, on a clean system, `pip install psycopg2` doesn't just work.