Point taken, though there's no reason the package manager couldn't change the environment variables based on the directory it's told to use for the install. The pieces are there, but nobody's really taken the time to put them together. For the filesystem, some simple bind mounts would probably suffice, though you'd still run into some permissions issues I imagine.

EDIT: It seems to be significantly easier on dnf, to the point that it could be trivial to add full support for home-directory installs.