Add a flake.nix file and you get completely reproducible program versions if you really want.
But I know that you meant it as a rhetoric, so my non-rhetorical answer would be that python3 is almost universally available on distros that are not minified deliberately (containers).
> python3 is almost universally available on distros that are not minified deliberately (containers)
Writing control scripts for use inside containers is currently my biggest application of BASH scripts. What makes BASH (or other shells) handy is the lack of supporting files that are needed - just copy in the script and you're ready to go.
I was going to say something similar. I try to avoid adding anything to my final images. Each extra dependency is something else to update, more surface areas for attacks, another thing to justify, etc. Much rather just use the slimmest base and the tools already available.
The various sub-versions of bash are a lot more compatible than the sub-versions of python 3. Just today stuff was breaking on me because of differences between python 3.6 and 3.8
It depends on what features of BASH that you want to use. If you target v3 (as I usually do), then that or a newer version will likely be already installed. There's very little issue with backwards compatibility.
But I know that you meant it as a rhetoric, so my non-rhetorical answer would be that python3 is almost universally available on distros that are not minified deliberately (containers).