So now you want to discuss prior consumption? We should stick to horticulture.
It is a weed hence the title.
However, similar to many other drugs like alcohol or cigars or coffee, much of the best marijuana needs very specific and careful care. It is not as simple as tossing some seeds on the ground and reaping what you sow. Sure you might get something but much of the hash requires good conditions, climate and care.
So, sure you can grow it on your own. You can grow your own tomatoes too. But if you want good tomatoes, you need to hit a store which can give extensive care to the crop it grows.
And we can also tax seed exchange if you like. But for most of the horticulture, money will need to exchange hands at some point.