Even if the actual growing isn’t hard, you need to plan it months in advance and there’s work involved in setup and harvesting. That’s far more inconvenient than just opening an app or driving to the store. It can also be risky (there are still plenty of confusing rules around growing for personal use, and guerrila growing is illegal) and even meticulously attended plants have variable yields. The only way for it to substantially affect profits is if a non-trival portion of customers prefer the cost savings to inconvenience, and, imo, the evidence points strongly against this. Heck, at my local despensery pre-rolls far outsell flower and the only work involved there is grinding and rolling!

Also, I haven’t read that particular book, but the growing difficulty depends on the variety. I can personally attest that growing cherry tomatoes is extremely easy (they’re commonly recommended as a “beginner” plant.)