I understand your point. And it is a valid one, especially if your goal is to control as much of the grow you can.
But understand, many people who'll grow might not have the time to compost good soil. They might not have the vitality to turn the soil periodically, and they might not have the room to compost. Besides, it is just one more thing to go wrong.
I got my 'designer' soil for less than $20 for a 1.5 cubic foot bag, plenty enough for 3 plants, or hopefully 6 or more ounces. Which works out to about 3 bucks an ounce.
If you are a student of the gold rush, you'd know that the people who got rich weren't the ones digging the gold, it was those who supplied the miners. I think it is the same for legalized MJ, the growers could be squeezed by bigger growers, but those who fill niches, like soil, can make money.