Most are mouse first programs. Developers know that a majority do not own a pen tablet or something similar. There’s a reason why brushes in programs have smoothing, so that you can get nice curves even with a mouse.

Moreover, I would call GIMP’s experience with pen tablet relatively worse than using a mouse.

The reason why pen tablets are better in most cases is that you can use real drawing skills within those programs, like pressure and tilt, which cannot be achieved by a mouse.

Autodesk sketchbook works pretty well when using a mouse.