It happens with a lot of stuff. Chocolate candy bars in gas station kiosks are always the same 5 brands. Anybody _could_ make chocolate bars, but the big players make deals to control retail, so you don't get a choice of other candy bars, in the situation you'd typically buy one.
If I control the conditions under which you buy something, I can control what you are going to buy.
Because small chocolate manufacturers don’t have the scale to supply massive distribution companies. An little retail gas stations don’t have any incentive to go make deals with boutique manufacturers. That would be madness for a gas station owner to go to individual manufacturers for specific items. And the manufacturers would hate it. Dealing with hundreds of thousands of small orders would be a logistical pain. That’s the reason their is a distributor layer for those sorts of products. If customers at gas stations started demanding Valrhona chocolate, they’d ask their distributors to start getting it. I’d buy it, but most people at gas stations aren’t going to care about a $10 chocolate bar.
The monopoly is in the illusion of access to choice on shelves. The pharmacy markets is dominated in the US by 2 players - Walgreens and CVS. Heck they even have their own generic meds competing at the same prices
