The issue is progress is difficult to report. If I've got Process A displaying my progress bar and Process B doing the actual work I've got to have some sort of bidirectional communication between the two. It's more difficult if B is calling independent processes in the background. They need to report their status/progress to B which needs to broker those updates and pass back up to A.

It's not an intractable problem, just tricky and annoying. If a progress bar is a tiny fraction of an app's lifecycle it's a lot of work to make it accurate for little payoff.