IMHO, the short answer is that Bitcoin is a distributed network. If you want lots of people around the world to work on the same problem, you need to parallelize it one way or another.

This question has already been asked in many ways. If you can limit mining to a single network node (CPU), dedicated miners will just set up multiple nodes for themselves. If you want to limit mining power per person, you've just thrown away the whole idea of an anonymous distributed network. Also, do you want to ban people from working together towards a common goal?

Moreover, limiting generation per person throws away all incentives for developing the network. In its current form, the network is stronger against attacks, because people are rewarded for spending computational power on it. In other words, who is going to do any work, if you just give everyone a big chunk of money?