I still don't understand why you can't really use a powerful CPU for mining. Is a GPU really more powerful for these calculations?
Short answer: yes.
Why don't you use just a really powerful CPU to play games? Because they don't handle image generation as well as a GPU does. No matter how good a CPU is, you still can't play a recent game without a graphics card. GPU's hardware is optimized for parallel tasks such as image processing. Bitcoin mining algorithm also benefits from this kind of architecture.