> Anybody who notices the lack of sh can easily install it
It's not about not having /bin/sh! It's about systems which have *a* /bin/sh that *isn't* bash. All that POSIX requires is that it implements the POSIX shell as specified. Some distros use bash, some distros use dash, some use a stripped-down shell that only supports what POSIX needs.
The problem is software which assumes that you can write bash-isms and then run them with sh. If the script starts with #!/usr/bin/env bash" or something, then the user can install b"ash as needed. If it starts with #!/bin/sh then it won't work on non-bash-native platforms at all.