I guessed this was due to licensing issues, and a web search shows,Apple has switched from BASH to ZSH as the default shell in Terminal.app for new user accounts…
Caveat Casual Catalina User:The Next Web said:Why does macOS Catalina use Zsh instead of Bash? Licensing
Newer versions of Bash are licensed under the GNU General Public License version 3 – or GPLv3 for short. This comes with several restrictions which could potentially have caused a few headaches for Apple further down the line.
Catalina's bash is probably version 3.2.57 or so, if that matters to you. The current version is 5.0.