Exactly. It's not that much different than tobacco or tomatoes like cooking pointed out or whatever. People don't want to go through the trouble to grow it when they could more than likely get a finished product of higher quality at a store. And as long as there is competition, the prices would naturally be affordable enough that people would just rather buy an 8th at the market as opposed to growing it themselves.
Govt. grown, or govt regulated pot growth would result in massive economies of scale. Unless you're some hippie, you probably wouldn't be able to make an argument that it's better to grow yourself than to pick it up at the store. The cost savings wouldn't be that much, I'd assume, and the time commitment would be tedious.
Bottom line, how many people are bootlegging alcohol in today's society? Prohibition wasn't the answer with booze and it's not the answer with pot. Small time growers wouldn't be able to compete with large scale production. Less money going to organized crime, and more of it going to our government.