Because I don't want to have to read and figure out each person's bash idiosyncrasies, bugs, etc. in pile of undocumented scripts to add a new task that runs a new test case. Just gives you a framework to put all your tasks in one file, document them, and find and run them easily.

If bash works for you, stick with it. It does not work with large teams and people who aren't deeply experienced in all the footguns of bash in my experience.