Once in a long while (long enough ago that I can't remember), you'll find yourself critically low on gas, and maybe up in the mountains, and the only station within 30 miles is a tiny one with one 50-year-old pump with a reset crank on the side, so you have to pay inside. It's rare, though.
The armed robberies are typically in stores that sell lots of beer and lottery tickets, and therefore have lots of cash on hand. Most people that use the "convenience stores" use a welfare "food stamp" card to buy their ready-to-eat fish, chicken, and potato wedges, but they have to use cash to buy beer, wine, and lottery tickets.
So the bad guys know which stores those are in their city ... out in the country, there's much less of all of that, plus the consequences of trying an armed robbery could be ... worse.