I'm pretty certain virtualenv just changes some environment variables.
I found a writeup [1] on how to write your own virtualenv. It doesn't look like there's much overhead there.
I suppose one could justify not using virtualenv if they're on a system with limited resources or network access. I've known people who spent hours building a compiling python 2.7 from source on embedded systems because they rolled their own distro, and in that situation it would make sense to not want to mess with virtualenv if you just spent hours getting pip working.
I found a writeup [1] on how to write your own virtualenv. It doesn't look like there's much overhead there.
I suppose one could justify not using virtualenv if they're on a system with limited resources or network access. I've known people who spent hours building a compiling python 2.7 from source on embedded systems because they rolled their own distro, and in that situation it would make sense to not want to mess with virtualenv if you just spent hours getting pip working.
1: https://www.recurse.com/blog/14-there-is-no-magic-virtualenv...