Maybe there isn't that much money in it? The stuff does "grow on trees" after all.
There might be lots of weed stores for the same reason antique stores and cigarette shops proliferate - they're cheap to set up.
There's a ton of money in it. The arbitrage is that one farmer can produce hundreds of pounds of product but cannot possibly warehouse and sell it all themselves. The resulting wholesale prices make setting up a shop that charges retail prices an incredibly reliable way to make money.
There is no mechanism, as there is in food, to support farmers or to control consumer prices. There is also no government funded free marijuana program. It seems like it would have analogs, but marijuana is truly a unique market.