The assumption is that, if you were a genius, you'd be able to come up with that eureka spark in the time allotted in the interview.

I won't discuss whether this assumption is correct or not. But i will discuss that even if you hired such a genius, it would not ultimately make the company any more money, unless you could put such genius to great use rather than grunt work (which a genius would do no better than the grunt - thats why its called grunt work).