At the speed of a falling jet that's about to crash, your limbs would litterally be ripped off trying to jump. If you jump from a high altitude, oxygen is a problem.

Fighter jets have an ejection system (along with the actual seat) that stabalizes the pilot so that they are ejected upward properly so that limbs don't get ripped off and the actual parachute deploys at the 'right' time. Oxygen is also provided. This is all with in conjuction with appropriate training.