There's a lot more to the weed market than just plant, harvest, sell. When I was in Seattle, the legal weed stores often stocked close to a hundred different strains of the plant alone, plus a huge array of food, candy, beverages, accessories, etc. Selecting your inventory and dealing with suppliers takes knowledge and skill, just like managing a grocery chain.

A street dealer who may, on a good day, have half a dozen random strains available may have a small head start, but there's a lot more to it than that.