Nobody is signing a national contract for gasoline. It all comes from the local refinery, or in some cases a pipeline to a slightly more distant refinery. The final shipping is by truck and it's almost never the case that you can find a different place that you could drive a truck to that's cost effective. Shipping guests. A few more miles by truck, it's only a few dollars different, and so it doesn't cost much difference per gallon. However, when we're talking 100 miles, now you're talking about significantly more money and that shows up in the pump price. Worse, even if you are willing to pay the price, every state has different specific rules for gasoline and so there is a very high odds that only your local refinery can make the correct gasoline for your local area.
What national chains can do is specify the additive that the refiner adds. Anyone other than a national chain doesn't have enough profit to be worth trying to build their own additive package and takes the standard additives the refinery has, which are almost always a good package anyway.
What national chains can do is specify the additive that the refiner adds. Anyone other than a national chain doesn't have enough profit to be worth trying to build their own additive package and takes the standard additives the refinery has, which are almost always a good package anyway.