I had an acquaintance who grew some weed for personal consumption back in the day. To suggest that one need only stick a seed in the ground is pretty naive.
Plant. Determine gender of plant. Separate male from female plants. Grow plants. After the plants bud and mature, cut and hang upside down to dry out, and concentrate the active ingredients.
Not hard, but requires a level of effort most would be unwilling to undertake if readily available at the DopeStore (tm).
It wont bring in any taxes.