Why are all restaurants in airports garbage?

Any time I’ve flown and sat down for something to eat before my flight, I’ve had a sub-par meal. I’m not fancy or anything, but I do like a good meal. No matter the city (large and small cities around the world), the style (a chain or an independently run place) the food is always low quality. Why?

ETA: I am really appreciating the comments saying what airports have GOOD food - makes me want to travel there now!