I prefer hot meals too, but I think the reason cold chicken (or salmon which is sometimes offered in the same format) is cheaper for UA and quicker to serve is because it doesn't need to be separatley packaged for heating and is already pre-plated. There may also be some production efficiencies if the same cold chicken is used for salad dishes.

FWIW, I don't like midcon lunches either...especially when they are served on flights like the 3:50pm SEA-ORD which is really flying right through dinnertime!