Personally, I think an operating system should have the basic tools for the user to be able to do very basic things that the average user needs and/or wants to do on a computer. This is why Windows has MS Paint. Is it a top end photo editing tool? No. But it lets you play around a little bit if that's what you want to do, and then later, if you decide you want to, you can get something better.
GIMP is just too much for your average user who might just want to fool around with image editing, and honestly, it shouldn't be a big issue to install it yourself after OS installation.