Madame L knows you have all wondered the same thing. And here's the answer, in Conde Nast Traveler, online:
It's partly because after the food is cooked and refrigerated and re-heated, it loses some of its flavor, which is like, Duh!---what we all knew, all along.
But it's also because the cabin air and atmosphere decrease our perception of saltiness and sweetness.
So we can't blame it all on the airlines. Though there's still plenty to blame them for.