So, it's the process. California's weed is cheaper than Illinois', it's a different process. A different set of protection taking a different rake off the top, so each step of the process.
First, the growing licenses are limited. When Illinois opened it up legally, I looked into it. I certainly have the room, I'm close enough to transportation hubs and markets, and honestly, I can think of worse ways to make a living than growing marijuana as a cash crop if it were on the up and up and the product is profitable. I got as far as the application to apply for a license itself costing six figures. And this is to apply for a very limited number of spots, I think there were 15 or 30 in the first run. Also in order to apply you needed to get in place a huge amount of infrastructure and security arrangements. And this is in order to produce a product that is not rocket science, does not require a ton of indoor space, and is affordable to buy the apparatus for. But, it is a high effort/care/futzing enterprise to do well. So passionate people will wind up being very good at it. Also, it's really not that hard to grow a bunch more than you can smoke yourself. The heaviest smokers I know that use recreationally go through in the neighborhood of a quarter oz a week. There are people who use a lot more, but most users use much less. Plants can yield a pound each every lets say ~4 to 8 months. I'm generalizing, I don't have any experience, and I'm guessing like most commercial horticulture, knowledge comes with practice and then comes skill, which matters a lot for end yield and efficient use of input costs.
Somebody growing for themselves with time and money for the hobby can probably smoke out themselves and a dozen of their friends on average. If they can get some money for their hobby, it's all the more likely they do it. Which is why that's not allowed. Vinny already told you there's only two lawn services allowed in this neighborhood and don't you think it would be a shame if something happened to your nice bum bum in the penitentiary? And that's only step one. You now have storefronts that will only accept cash that are high risk robbery targets that have to hire their own armed security, which is expensive. You also need a lot of government approvals and licensing for the store. You now need to pay people at least minimum wage to work the store, because the entities that get all the licenses are not mom and pops, they're owned by investors. All the infrastructure increases the take at every step, which then increases the total amount taxed at the end. Whether or not all the regulation is a good idea, a bad idea, or potato... this is all taxation. There was a rebellion when we did this to whiskey in the 1700s. We* crushed them then, this fight's been over for a while.
*And when I say We, I mean with George ****ing Washington We crushed them.