When your script will be running on customer machines, you often don't have the luxury (and sometimes it's not technically possible because it doesn't exist) of installing bash into the environment.

What prevents you from shipping bash with your solution?

These scripts are often used to install the application, so that introduces a chicken-and-egg situation. How can I install bash before running the installer?

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.

How can you install bash without using it? With sh as bootstrapping shell.

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!

Time to find better customers.

You don't always have a package manager, root access to even be able to install anything, network access, etc.

"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.

That's the sort of yak-shaving I will never do. I'd rather just not use such a primitive unix and/or be in a job where one of pdksh/ksh/zsh/bash is not available at all.

Well it’s good that you have the choice to; but it’s also not all that difficult to understand that not everyone who needs to use your script, has that choice.

That's cool. We all decide for ourselves what markets we're willing and unwilling to address. But the market that involves "primitive" unix is lucrative and that can be worth it.

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.

It's not usually hard to do, no, but it is >130K of C alone, excluding whitespace, comments, tests, examples, etc. I don't want it on my system from a security perspective alone.

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).