A lot of open source graphics software is made by programmers who also need to edit images sometimes. Both the lack of UI polish and featureset choices make more sense when looked at from that angle.

However, a lot of the criticism that gets thrown at these programs is also a bit unfounded. I regularly see people dunking on GIMP for not being a pixel-perfect clone of Photoshop for free. There is more than one way to design an image editor, and inability of some to learn another is really a user issue. GIMP could be better, but it still can and should be GIMP.