You can always install from source and pass --prefix and --exec-prefix options to ./configure. Virtualenv is not strictly necessary to run in an isolated environment.

And as for whether virtualenv should have been part of the OS... it already is in some cases, to the extent that any userland tool is part of the OS.

    $ uname -o
    GNU/Linux
    $ which virtualenv
    /usr/bin/virtualenv

Does it really matter whether it was the python developers who got there first?