Cell towers don't need to be a freestanding structure. There are tons of micro towers hanging out on existing buildings.

I've seen them tied to the side of existing buildings, and even masquerading as a tree :) Something I've noticed is that it seems the denser the population, the lower the towers are.

It's logical: the higher the antenna, the larger the area it has in its line of sight. If your base station is going to serve a large area, you want it as high as possible. If it's only for a small one, not so much.

You don't even need a tower per se -- you just need smaller antennas covering the terminal building connected to a fiber backhaul.

Not to mention that airports can also be rich RF environments, and extra sources of noise pose extra complexity.