Yes, the Cell is powerful, but you fail to consider that graphics are only one part of what goes into a game. AI, sound, user input, and network operations also need to be considered. Given that, in a normal system architecture, a general purpose CPU handles all of that, and sends off instructions to a specialized GPU, the Cell architecture is radically different from what people are used to. That's what makes it hard.

I sincerely doubt that there are many(if any!) developers programming directly to the Cell processor. They're probably programming to Sony's SDK, which is, reportedly, very difficult to work with.