In general, because jumping out of a plane from 30,000 feet, will more than likely kill you within seconds (without proper training/gear).
"I'm supposed to jump out of this plane when we reach 30,000 feet. That's airline cruising altitude, 1,000 feet higher than Mount Everest and six miles above the earth's unforgiving crust. There's no flight hostess or pressurized cabin, which means that without a mask you would have about 30 seconds of useful consciousness before you would black out from lack of oxygen and risk severe brain damage from hypoxia. Open your chute at this height and it explodes, at which time you plummet to earth in a blizzard of confetti -- the air being so thin that you fall at twice terminal velocity, too much for a parachute to take. This is the earth's troposphere, and it is a very, very bad altitude."