Once upon a time there was a program called Photoshop, created by the Knoll Brothers. Twenty years and eleven versions later, it basically remains unchanged. Except for the damn bloody tabs.
It's not only the tool palette. It's the entire user interface, which has remain basically the same since the 90s, after its creation in 1988. But then again, why fix what is not broken? Maybe someone can come up with a better interface metaphor for manipulating images. Some people say that the Photoshop interface and workflow works because people are used to it. Maybe they are right and someone needs to break the mold, but everyone who has tried has failed miserably so far. And two decades is enough to come up with a worthy competitor-and please, don't even mention Gimp.