Why are airfield restaurants so common across the United States? It's not the food, it's the required flight hours needed to stay current on licensing!

Part of flying is you have to maintain currency, where pilots regularly fly and log specific tasks to stay eligible to fly.

If you’re going out and just trying to do some practice maneuvering, taking off and landing, or other instrument work, it’s an excuse to go out, do that, have lunch somewhere, and then train on your way back as well.

These restaurants are normally far less busy than others in town, as they are simply off the beaten path, without common city streets going past their parking lot, and instead being on the airport. 

San Diego's Montgomery Field has one, Casa Machado, and it's the monthly meeting location for the San Diego Mopar Club. On the other side of the runway, is a cool little restaurant, the 94th Aero Squadron

Typically these restaurants have excellent viewing windows for just sitting and watching the planes and helos come and go

https://www.atlasobscura.com/articles/restaurants-to-fly-to