A lot of (most?) useful work requires the kind of domain-specific knowledge that can only be built up over a lot of time spent working on a given team/product/codebase. "Renting" programmers here and there on an hourly basis might work fine for basic tasks, but would probably end in frustration for both the employer, who has to deal with a constant stream of random new people, and programmers, who are constantly bouncing from project to project.