In the case of Photoshop I think it's more that an established system with a large userbase can't significantly change its UI without significant negative feedback (see Office Ribbon).

The pallet interface is also what we're used to. Most people, were they to set out to build a pixel pusher app, would start with a basic pallet UI. Unless of course they were specifically aiming to get away from that.

Also, Photoshop is a perfect example of "The more painful the UI is, the more satisfied these users are".