I don't think it's for any particular reason, other than that nobody has done it yet. I would imagine that the response of most projects would be "PRs accepted"

I don't think it's for any particular reason, other than that nobody has done it yet. I would imagine that the response of most projects would be "PRs accepted", unless they consider it explicitly out of scope. I could see some projects not being interested in automatically installing versions using Pyenv. But I think most projects wouldn't mind the option to defer to Pyenv before searching PATH, or to otherwise configure a custom Python search path that could be derived from Pyenv and/or other sources. Note that Pyenv is itself basically a pile of shell scripts, and it doesn't work on Windows (although there is a Windows port of it). There might also be a general expectation that this functionality is too complicated to support in a way that makes everyone happy, and that users need to figure it out on their own, e.g. using the technique described in the article here. You can also go the other way and use the Pyenv-Virtualenv plugin.