To jump out, they would need to open the doors. There would be problems with decompression at above 10K.
You have to deal with people unable to use parachutes. Children, elderly, disabled, afraid of heights, and panicked.
There's an assumption an airplane remains level enough. If it's spinning or nose down, trying to reach an exit is another problem.
If jumping out ahead of the wing, there's a risk of getting sucked into the engines.
Parqchutes are bulky. Trying to get them out of storage and distribute them to a couple hundred untrained people is a tall order.
Putting on a parachute, correclty strapping it, knowing when and where to pull the cord, and knowing how to land without breaking bones, hitting tree branches, or ditching into water. These are all issues you can't teach during preflight safety instruction.
Overall, everyone would be better off staying put, not panicking, and hoping a plane and trained pilots can get everyone on the ground, safely.
If a plane can stay level enough for long enough to get people into parachute gear and out the door, chances are good that the pilots can land that plane, which significantly decreases the chances of injury to the passengers.
Building them into the seats makes about all those problems go away. But decreases the amount of seats you can fit on plane and amount of money made per flight and therefore is never going to happen
So, some kind of detachable roof that doesn't randomly detach when it shouldn't? This also doesn't solve the speed, air pressure, and cold problems for the people in the seats.