As John Hansman, the director of the International Center for Air Transportation at the Massachusetts Technology explains: "It’s actually very hard to cook at those altitudes...they generally are just doing reheating.”
In fact, everything about serving food to a sold-out flight of more than 300 patrons revolves around efficiency. Your food is even pre-cut, so flight attendants don’t have to supply you with knives. This is also a precautionary measure, because no one likes a knife wielding maniac, and especially on an airplane.
As Albright College professor Guillaume de Syon explains:
“Airlines have discovered that, if you also pre-cut the meat, you practically don’t need a knife,” he said, adding that "because it's been so overcooked, you can cut it with a fork."