This post from Daniel Weibel not only explains how macOS uses an outdated version of Bash, but also how to upgrade to the latest Bash via Homebrew.
One thing that many macOS users don’t know is that they are using a completely outdated version of the Bash shell. However, it is highly recommended to use a newer version of Bash on macOS, because it enables you to use useful new features.
$ bash --version
GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin18)
Copyright (C) 2007 Free Software Foundation, Inc.
The reason Apple uses this old version of Bash has to do with licensing. Bash 4.0 and newer uses the GNU General Public License v3 (GPLv3), which Apple doesn’t support. There are some discussions about this on Reddit.
Version 3.2 of GNU Bash is the last version with a license that Apple is willing to accept, and so it sticks with it.