Monopoly? I thought so too, until I found out that Walmart is the largest grocer. This is a merger of #2 and #3 (at least in the West - Safeway (Albertsons) isn't even #3 nationwide).
I rarely shop at Walmart, for a number of reasons, but on a couple occasions I've picked up some milk at Walmart, because I needed some and wasn't near my other grocery stores. Never again though. The last time was ridiculous: Walmart marked up every brand of milk to absurd prices much higher than other grocery stores, except for their "store brand" milk, which wasn't cheap either. I didn't even bother getting a full gallon, just the smallest I could find and got the hell out. I couldn't imagine shopping there every week.