Not installed by default on BSDs. Sometimes not installed in minimal environments, e.g. where BusyBox is all you have available. Maybe not installed by default on Unixes? Not sure
Also, we'd have to build and maintain our own just for platforms missing it, which increases cost and complexity. It's easier, cheaper, and less error-prone just to use a minimum common denominator that runs on everything.
Additionally, when your customers have strict policies regarding how applications are approved for use and have to vet and test everything you give them, it's best practice to avoid installing anything that you can avoid installing. Doing that minimizes the effort and hassle for both the customer and the developer.
Build and maintain your own bash or install script? You don't have to maintain it, it's just building which is already provided by original source build scripts. You can install it as "mysolutionsh", nobody cares as long as you link it to PATH or reference it explicitly. Still easier than avoiding bash everywhere but in personal setups. Yours already familiar with it, that's a waste!
"Just install bash" is not always easy, and is not even necessary when posix shell can easily do what most people use bash-specific syntax for.
For me as a developer, it's not a big deal. You can do all the same things, just in a slightly different way, and you have a script that can run almost everywhere.
Add in the bootstrapping pain it imposes due to autoconf and other stuff, I think there are many valid reasons to avoid it and choose another shell that is more auditable yet still has just as many eyeballs on it (e.g., mksh - the default on Android; dash - default on Debian; BusyBox - every embedded system).