You are comparing boiling an egg to making an omelet. You do know that. Don't you?
No, he's comparing making eggs for breakfast to picking up an egg McMuffin. Most people would rather buy something that someone else made than make it themselves (if the price is reasonable). There is an "economy of scale" that says it will be far cheaper (and more convenient) for one person to grow a lot and sell it than lots of people growing a little.
So the cartels, who already have large plots dedicated to growing the stuff, would continue to dominate the market. The average user would be perfectly happy to continue to buy rather than go through the trouble of growing his own (it's not like the first word that comes to mind when I think "pot smoker" is "industriousness")...
Sure you can grow ditch weed in your back yard but if you want quality bud you have to get into hydroponics and other advanced cultivation techniques. Few will bother if you can purchase it at the local stop and rob.