Linux in practice always comes with bash, but sh is not necessarily an alias; e.g. on the world's most popular distro:

    > readlink -f /bin/sh
    /usr/bin/dash

Also, BSDs and derivatives either have no bash installed by default (e.g. OpenBSD) or, in the case of macOS a horribly outdated one.

Finally there are good reasons to hate bash: Bourne shell is flawed but brilliant, whereas bash is a bloated mess of unpredictable and counter-intuitive behaviour that also tends to change between versions. It's also much, much slower than e.g. dash. Unfortunately even for pure shell scripting bash in practice is the way to go; posix shell (and dash) unfortunately lack a few things that are pretty crucial and very cumbersome to work around, like process substitution.