I think it's probably actually just because they want to support users who wish to have a "minimal" python installation. Python is used internally by the system and many applications, but that doesn't mean that features like venv are necessary for them. It does make sense not to install them unless needed.

That said, I think debian users should lean heavily on the apt versions of any python modules. I will usually install as many modules with apt as possible and then use `python3 -m venv --system-site-packages` to have access to them. That way there are usually very few packages I need to compile/install myself. Seems like a pretty good balance.