Yeah but it was the wealthy corporate alliance who had the droid armies. That meant something inside the prequels themselves, but importantly and especially feel like it means something in a show like Andor. Droids are purpose-built and have personhood in Star Wars to the point that it’s plausible to me that flexible human labor ends up cheaper.
Either way, it’s probably a Why Not? thing more than it is a real calculation. It’s not like we don’t have prison labor in our own world. the empire is evil, they’re trying to get as much out of everyone as possible