Serving a meal on an airplane has two purposes, one nutritional, one psychological. The nutritional purpose is fairly obvious - it's the psychological side I want to talk about.
Flying sucks. It's stressful for a lot of people (me included) and excruciatingly boring. It messes with ones sense of time, makes legs hurt and strains neck muscles from the whiplash incurred by looking for the drinks trolley.
In order to alleviate this, airplane food is specifically designed to take as much time to eat as possible. Everything in the meal comes individually (and diabolically) wrapped. A certain amount of high level thought is necessary to decide whether to eat the roll by itself, with the cheese or to dip it in the gravy. The tray it is served on is not quite big enough for one to eat comfortably, and is certainly not big enough to hold all the wrappers from the things to be eaten. The meal arrives in stages - first food, then drink, then coffee. It keeps you waiting for the next treat and keeps your mind off of your mortality. It's quite a good trick, really.
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Zerotime says re Airplane food: Plus, there's that feeling you get three bites into the chicken cordon bleu: "is this food, or cunningly extruded plastic?" Personally, I like airplane food and I was trying to avoid the whole chicken vs. cardboard comparison, but I suppose figuring out your dinner's base materials, radioactive or not, could occupy a few extra seconds.