So with a captive audience and no competition, there is no economic incentive to provide quality nor is there any competitive pressure on price so you can charge whatever you want with no fear of a competitor undercutting you.
Most stadiums now charge around $9 for a 16-oz plastic cup of cheap, often watered-down beer. Profit margins on stadium concession drinks (including soda) are well in excess of 90%. The margins on food items aren't too shabby either.
Captive audience + no competition = High Margins with minimal capital input and no incentive to improve quality of product.
Much of that margin of course, goes to the stadium as well as the concessionaire. And if the stadium is owned by the ballclub, as most of them are, then it is the owner of that ballclub who directly benefits by the arrangement.
Big, big money in concessions. That is why the owners want to still fill the seats because that is "unshared revenue" for NFL clubs. Most NFL revenues such as television rights and ticket sales (excluding luxury boxes) are equally shared among all NFL teams.
I never use the concessions at the stadiums though. I always go to a local bar before the game (or concert event) and have a meal there with a few beers. That is where you find the best food at somewhat reasonable prices. Then after the game, I might do the same if I'm still hungry.
Now that’s the way to do it!