because GIMP sucks, maybe, and also because most people are required to interoperate with the rest of the profession. i am very disappointed that photoshop et al cannot be run under WINE :(
I don't agree with you that it sucks.
Think not just about now or one year from now, but 10 years, 20, 50 years from now, 100 years? Open format and open source software is much desired for creation based software. There is the paradox of the making money for the open source software, and I don't have an answer for that. Seems like some kind of sliding scale payment will be needed for these software. May be also some kind of sliding scale payment for eventual money earned form the software up to a limit.