[robocat]

I wondered why Apple use an old bash version, Google said: "The reason that Apple includes such an old version of Bash in its operating system has to do with licensing. Since version 4.0 (successor of 3.2), Bash uses the GNU General Public License v3 (GPLv3), which Apple does not (want to) support."

[Aperocky]

You can easily upgrade the bash to 4+ on mac. Just have to do it yourself.