Earlier quoted context omitted.
Why would a software system that relies on bash not explicitly use bash for its scripts?
Bad developers. For example, I used Arch Linux for a number of years, where `/bin/sh` is a symlink to bash. Switching that to point to `dash` broke a number of programs both in the official repositories and the AUR. A lot of shell scripters can't spot a bashism, and just as many don't know that other shell syntax even exists.
Unless you want to define "bad" as "ignorant of some facet of computing" in which case we all fit.