Some people have the budget to spend up on the 975, and they don't want to spend time troubleshooting crashes and BSODs by overclocking. Not everybody is an overclocking gamer; some people have serious media applications that benefit from the 975 over the 920, and overclocking can be intimidating.
If I have the budget, then I'd be an idiot to buy the cheap processor just to tinker with BIOS settings to replicate the performance that I can get from the 975 out of the box. Having some money doesn't make somebody an "idiot".