But you can compare to, say, Baltimore, where there's barely any surface parking around the convention center there, despite the fact that it's right next to a major sports stadium.

Sure, you can make that comparison, I just don't know what useful conclusion you'd get from it. Maybe property rights are stronger there. Maybe nobody offered enough money. Land in Atlanta has historically been relatively cheap compared to New England.

I have a pet theory that states that land values are higher on the coast mainly because sprawl doesn't have as many places to go, so it concentrates in smaller areas. Atlanta was historically a rail hub, exacerbating sprawl because transport could always move people farther outward. Picture pouring money onto a map, and walls are anywhere it's infeasible to develop the land. The money will spread out where there's no natural barriers. It will pile up if there's a lot of them. Money piling up means increased property values.