It is even worse on MacOS, because Apple bundles the BSD versions of common Unix utilities instead of the (generally more featureful) GNU versions. So good luck writing a Bash script that works on both MacOS and Linux...
Anyone using MacOS as a unix platform is installing updated tooling with brew or similar.
Half sarcastic with that one
Also, zsh is not installed by default on most distros.
Why they went with Bash 2 as the defualt shell is beyond me. I always switched to and used Zsh which had a more recent version. Now I'm also using it on Linux and FreeBSD, because I want a consistent shell.
The reason was the same as here: bash moved to GPL v3.