Most planes fly at altitudes like 35,000 feet. If you tried to exit a plane at that altitude you would die almost immediately. The air pressure at that height is so low that if you held your breath, your lungs would explode almost immediately. Even if you were to breath out REALLY quickly, there isn’t enough oxygen at that altitude to keep you conscious for very long at all. So you would lose consciousness. Then you have that small problem of pulling the chord to deploy your parachute – not possible to do if you are unconscious… If you deploy it as soon as you exit the plane the lack of oxygen would cause your death on the slow descent.
You are also looking at temperatures 30 degrees below zero at those altitudes – the eyes, nose and mouth would freeze over almost immediately.
You would also be exiting a plane flying at around 150 miles an hour or more – assuming you could get out of that plane, that air pressure on you would kill you immediately, too. Most planes fly at more like 500 mph.
Now, just think of the logistics of carrying them when a parachute can weigh up to 40 pounds – one of those per passenger – that increases the weight of the plane, increasing the fuel used and therefore the cost of tickets, for a benefit that would probably kill most passengers trying to escape the plane.
The other thing is – most airplane accidents happen during takeoff and landing at heights that would render a parachute useless – most main parachutes require 600 to 1,200 feet of freefall to open and slow the person down.
So the added cost of doing so, lack of training of the passengers in how to exit an aircraft and land safely, plus all the issues of getting out at high altitudes make parachutes not worth the cost.