Because either need skilled personnel. Most have ATMs, so a bank is not needed.

Gas-station convenience stores stock items that are a good mix with the kind of customers the gas-station attracts and the situations they are in. ATMs are a good match, as are easy-to-eat foods, road maps, cigarettes, incidental automotive items, selected over-the-counter pharmaceuticals, condoms, etc.

Prescription drugs would be a bad fit because the customer's need to return to pick up a filled prescription would not necessarily co-occur with his car's need for more gas.

I'm looking for a gas station, and would much like to create a store other than a convenience store. Unfortunately, I doubt I would be able to so that.

The ability to do this depends alot on the location, size of the building, amount of parking at the particular gas station, what the gas supplier or property owner will allow, and many other factors.

Most importantly, because this has been not been done frequently before, customers will not buy items other than convenience store items at a gas station. Perhaps if the gas station bordered a retail plaza space, people would shop in the retail store. However, I've rarely seen such gas stations.

Is this even an idea?

I've seen some larger stores (CostCo, WalMart) that have little fueling stations on the outskirts of the parking lots. I believe the one at the local CostCo is members-only and maintains consistantly lower costs than all other stations in town, so it works well. The ones I've seen at WalMart work, too, since people often make long trips just to get to the WalMart or they visit it daily for other shopping needs. Really, though, I think the convenience stores can't be beat as a small footprint pairing for fueling stations.

So, let me get this straight...you believe that Sunoco could just open a Bank of America branch on their property? How about if they just add a nuclear reactor?