Probably because grocery store margins are low, and the number of employees for a small one vs a large one is pretty similar, so it's cheaper and easier for the company to build larger ones.
We do have "convenience stores" some of which are effectively very small grocery stores (produce, fresh meats, etc). But usually significantly more expensive.
Convenience stores aren't really more expensive small grocery stores. I'd challenge you to regularly pickup the ingredients for a tasty healthy dinner from a typical US 7-Eleven.
If you're lucky they may sell not quite ripe bananas at the front counter. Otherwise the healthiest is going to be cheese from the cooler, orange juice, and tortilla chips.
We do have "convenience stores" some of which are effectively very small grocery stores (produce, fresh meats, etc). But usually significantly more expensive.