There are four reasons why you don’t find a parachute under your seat on commercial airlines:
Parachutes are bulky, heavy and expensive. They would not even fit under your seat, they would occupy a lot of space, and add a lot of weight. They would also need regular inspection and repacking. To provide several hundred per airplane would add significantly to costs, making flying much more expensive.
Passengers are not trained to use them. Without a minimum of training most people would not even be able to strap the parachute on correctly, never mind open it and land safely. Even on the ground and with plenty of time this is not easy. In the confined space of an airliner and in a high-stress situation it would be even more difficult.
There isn’t a convenient way to jump out of typical airliners. You would need to redesign aircraft with a special jumping exit. Just jumping out of normal side-facing airplane doors or emergency hatches you would probably hit the wing or the tail. You would need to install a ramp on the rear of the cabin.
There are very few situations where it would save anybody. You would have to be in a situation, in daylight, over land, where there is no hope of a landing but plenty of time to get everyone out. I can only think of a single case in the entire history of civil aviation where it could have been useful — The United Airlines Flight 232 case —but then only if the aircraft had been redesigned with a rear exit. Even in that case, jumping was as risky as staying aboard.
