In case you failed sixth-grade science, the air at 36,000 feet isn’t breathable -- this is why airplanes are pressurized. Even still, the cabin air has far less oxygen than you would breathe if you were on the ground and, though some studies have shown booze doesn't have much effect on your BAC, it can still be metabolized faster and exacerbate the effects of altitude sickness.
You can’t board your connecting flight
And since those two light beers on the ground may have hit you more like a couple of Four Lokos at altitude, you’d better hope that two-mile walk through DFW to your connecting flight clears your head. Gate agents are required by law to stop you from boarding the plane if you appear intoxicated. Which is entirely subjective.
You might get arrested
You get a little belligerent in a bar, they throw you out. Maybe they call you a cab. Do it on a plane? Well, they have no option but to have a couple of nice gentlemen in uniform play the part of your parents when you flew home from college in the '90s, and greet you at the gate. Only this time, nobody's stopping at Cracker Barrel on the drive home.