They do symlink /bin/sh to be fair, and that's very often good enough for a lot of scripts. That's what I usually do if I don't need anything bash offers.
Thing is, a few years ago when Debian changed its default sh from bash to ... either ash or dash, I forget which, I got into the habit of always writing `#!/bin/bash` at the top of my scripts, in case I didn't realize that something I was using was a bashism not found in classic /bin/sh. So if I used Nix (I don't, since for my particular use cases the juice isn't worth the squeeze), I would get seriously messed up by that.