Rare would not make more money by being owned by Activision. You think Rare sells their games alone? It's Nintendo that does the advertising for Rare, it's Nintendo that gives Rare the franchises to sell their games. Plus, Nintendo gives Rare more support than Activision ever could.
Nintendo basically funds Rare and everything they do. ActiVision doesn't have the same kind of money, and they can't give the same kind of help, support, and stuff like that. Look at StarFox 64, took off because it had a rumble pak. As a 3rd party, that wouldn't happen. They also wouldn't get the hardware as fast, and wouldn't have the help of Nintendo on their games, like the guidance of Shiggy and stuff. Hell, didn't Nintendo pay for Rare's facility?
Oh, and let's not forget, Rare can't leave until their contract is up
