Apple added /bin/dash in Catalina, and the intention is probably to eventually to use dash as the implementation of sh: https://scriptingosx.com/2020/06/about-b

Apple added /bin/dash in Catalina, and the intention is probably to eventually to use dash as the implementation of sh: https://scriptingosx.com/2020/06/about-bash-zsh-sh-and-dash-in-macos-catalina-and-beyond/#:~:text=Apple%20also%20added%20dash%20(but,the%20interpreter%20for%20sh%20scripts https://scriptingosx.com/2020/06/about-bash-zsh-sh-and-dash-.... As for why they haven't done it yet, I'd guess it's just to preserve backward compatibility with old scripts that rely on quirks of bash's sh compatibility mode.