Because creative FOSS is usually terrible, their methods demolish the flow of an average creative worker with constant "No, it doesn't work like that "-type interruptions.
Intuitive functionality is never a priority and never done right. If anything it seems to be viewed as sacriliege to sacrifice clean and simple code for the base instincts of an ignorant user.
Gimp logic is a bit different from whatever logic a regular user is used to. It's easy to work with once you familiarize yourself with it, but if your intuition is trained on Windows products, you will need to readjust.
It's like a traditional martial art, you must first master arbitrarily complicated forms before, maybe, being able to make something useful of it it in ten years.