It would be nice if Python could actually support this.

But 'venv detection' is conventionally (everywhere) done by a stupid, never-designed involving inspecting argv0. At the same time, most Python packaging tools make various assumptions like "if I'm in a venv, I can write into it", or "there will only ever be one level of venvery, so I'm not in a venv iff I'm looking at the system Python".

Nesting/layering venvs has never been supported by any Python dependency management tool except for experiments that were rejected by more senior people in the community, the maintainers of major dependency management tools, etc.

Same kind of thing goes for allowing multiple versions of a dependency in the dependency tree at runtime.

There's little appetite for fixing Python's dependency management issues in a principled or general way, because that would require breaking things or adding limitations. You can't have dependencies partially managed by arbitrary code that runs only at install time and also build good packaging tools, for instance.