It's because people are fucking retarded.
Even in a death spiral, chances are good the pilot will somehow unfuck it.
If in a death spiral, you have an excellent chance of jumping out and getting your dumb ass sliced in half by the wing or suiciding by the formerly last working engine.
If you are hurt and die using a safety feature, they can get sued way more than if you die in your seat.
People would decide to be "lol so crazy haha prank" and grab their chute and bail at 50k, causing everyone else's eardrums to follow the retard out the door as he and they lose consciousness.
If they spaced out properly, only five or six would get out before crashing by the time they knew they had to.
It's harder to fly a crashing jet with a door open and ballast shifting/jettisoning.
Did I mention getting hit by the wing/engine/tail even if you left the back because obviously the plane may be fucking up?
Stewardesses would be failing at trying to stop entire plane from panicking and crushing each other to get to the front of the line.
People overcrowding, getting tangled, deflating/popping each others' chutes, etc
Stewardesses made to choose between doing their job and dying while trying to keep a calm line and lying to the ones in back so the ones in front could live while standing in non-reassuring puddles of their own piss, or jumping out and leaving the sheep behind only to be disgraced or even face charges for abandoning people.
Also littering.