Photoshop is feature-complete, and the features are implemented well. For example, compare the speed of applying a 200px-radius Gaussian blur in Photoshop vs. Gimp.

To make an analogy with more hands-on trades: buying Photoshop is like getting the entire Snap-On tool catalog for $500. Buying Pixelmator is like getting 1/4 of the Craftsman tool catalog for $50. If you can spare the extra $450, you get a lot for your money.

Professional mechanics spend $xx,000 on Snap-On tools because they can justify the cost. Most DIY mechanics buy cheaper tools. But if the Snap-On tools only cost $500, why not get the best?

(edit: GIMP is like getting the entire Harbor Freight tool catalog for free.)

Also, most instructional material for image editing assumes Photoshop.