Its because GIMP's UI is designed for users that have more traditional Linux style window managers(tiling, focus follows mouse etc.)
Whereas most other programs, including the Adobe suite, are designed for the more underpowered window systems in osx and windows. That's why it is painful to use in those environments and environments like gnome which follow a similar design philosophy.
Comparing Gimp to X-Windows window managers is pretty apt, actually:
"In summary, ICCCM is a technological disaster: a toxic waste dump of broken protocols, backward compatibility nightmares, complex nonsolutions to obsolete nonproblems, a twisted mass of scabs and scar tissue intended to cover up the moral and intellectual depravity of the industry's standard naked emperor.
Using these toolkits is like trying to make a bookshelf out of mashed potatoes.
- Jamie Zawinski"
Well, then I guess its just a happy accident because the separate windows for different tools makes it a lot more convenient for with weirdos like me. So thanks a bunch, even if you didn't mean it!
Whereas most other programs, including the Adobe suite, are designed for the more underpowered window systems in osx and windows. That's why it is painful to use in those environments and environments like gnome which follow a similar design philosophy.