Airports are massive pre-planned industrial facilities with utility connections to match. Getting chargers there should be easy mode compared to most businesses.
The power requirements were not drawn up with the car rental agency in mind. Maybe you can get the airport to cooperate and ask the utility to expand capacity - how many years of planning do you think that takes?
They have the added challenge of people dropping off cars at a different place from where they started, so they can’t necessarily say “we’re going to start with EVs at our new facilities where we can design that capacity from the start.” They needed fast charging everywhere for this to go smoothly.
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.
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.