Because everyone wants our taxes to crack down on every herb garden in the neighborhood that evades taxes! specially the heirloom tomatoes! those creates multi-generation lost taxes oportunities!
everything that is somewhat not-ilegal will have small scale informal market. Friends grown their own tobacco, roast their own coffee... and all sell a little on the side.
how dense you have to be to think a literal weed would be different?
Down the road from where I live, there is a family with a veggies stand in their front yard. They sell anything they grow in their modest greenhouse but don't manage to eat. Leave a few dollars in the jar and take a basket of tomatoes, cucumbers, etc.
When cannabis is normalized in your community and people grow it for personal consumption, there will invariably be some left over that gets sold to family, friends, neighbors, etc. These sort of small local transactions are totally outside the purview of any government. The family down the road from me doesn't have a license for their veggie stand, I doubt they pay taxes on the proceeds, and nobody gives a shit because this is the way it has always worked and it's a generally pleasant and agreeable arrangement.