It's simple, because as americans we are all some what lazy, it's easier to have someone else do the work. Not only that but it would be cheaper to buy in the store than growing yourself. Do you think you could grow your own tobacco and make it into a usable substance cheaper than lets say Philip Morris, I don't think so.
The only people who would be growing are the people like us who want good quality marijuana, not the stuff Uncle Sam is going to give us. Hey, either way we all benifit, well maybe not the dealers but normal stoners like us and of course the good old U.S.A.
peace, redeyed