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.
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.
Also installing everything with pip is a great way to enjoy unexplainable breakage when a Doesn't work with v1 and b doesn't work with v2.
It also leads to breaking Linux systems where a large part of the system is python code. Especially where user upgrades system python for no reason.