"Why can't they grow it themselves?"
Because it is illegal and it's much easier to get caught growing it than it is just smoking it and the penalties are much worse for "manufacturing." Besides, most don't need as much as they'd have if they grew it, and hardly any would want to go to all that trouble anyway, even if it was legal. It's cheaper than beer. Why go to all the trouble of growing it? Most people would rather take the easy way and just pay a few bucks a week or month or whatever for what they smoke, or just smoke their friends' pot, or both.
First, there's a 150,000 medical marijuana patients who can grow legally under California law plus however many "caregivers" there might be. Second, if they don't grow it then they aren't "just smoking it", they're probably buying it from a drug dealer.