I don’t have a source. It just seems obvious to me they wouldn’t bother with the cost of maintaining that infrastructure.

Do you have any reason to believe car rental companies maintain onsite fuel stations? Have you ever seen one? Why don’t they let customers use it?

There are already gas stations near airports. If a car needs gas they pay someone to drive it there and fill it up. That’s the source of your inflated prices.

I haven't at an airport, but I have at freestanding ones. Having fuel delivered to a tank with a pump isn't really that involved as infrastructure goes. You're just buying the fuel wholesale instead of retail. You don't need the same size underground tank as a gas station, but you can hold more than enough to fill up whatever you need to from a fleet of cars. As for why they don't let customers use it? This is obvious. They'd rather overcharge you. And they know nobody's going to do something dumb at the pump if it's only their own team doing it.

Plus, often when I rent a car, it isn't full. I'm sure they still charge the person "to fill the tank," but they don't actually fill it, so the total they'd have to store on-site is even less than if they were actually filling the cars.

I also assumed they had somewhere to store siphoned fuel, because every car I've ever driven has at least a gallon before the needle moves off of "F," but rental cars drop below F immediately. I've always assumed they were siphoning out a gallon or more any time someone did return the car all the way full.