If you install a package in a fresh environment then it does actually get installed. It can be inherited from the global environment but I don't think disparate venvs that separately install a package actually share the package files. If they did, then a command executed in one tree could destroy the files in another tree. I have not done an investigation to look into this today but I think I'm right about this.

In better designed systems than python they do. To share them with python you need something with dedup. Eg BTRFS ZFS

Python's venv design is not obviously unintelligent. It must work on all sorts of filesystems, which limits how many copies can be stored and how they can be associated. More advanced filesystems can support saving space explicitly for software that exploits them, and implicitly for everyone, but there is a cost to everything.