Mostly not enough oxygen in the air at the low pressure up there.
Airplane cabins are pressurized because humans cannot breath at a very high elevation.Aircraft are pressurized to allow them to fly higher. Without pressurization, the amount of oxygen in the air would not be sufficient for humans to breathe.
high altitude chambers
Helicopters don't generally fly high enough for cabin pressurization to be necessary.
Airplanes travel faster at higher altitude. But pilots don't travel better at high altitude. Above 10,500 feet, people start to be affected by oxygen deprivation: tunnel vision, slower reflexes, slower reaction times, impaired judgement, etc. Aircraft are pressurized so people don't fall asleep and/or die at high altitude. At regular commercial airline altitudes between 10,000 and 40,000', all aircraft have to have pressurized cabins. Pilots wear oxygen masks (with the gas turned off) at those altitude in case there is a pressurization failure or a rapid cabin decompression. If one should eventuate, they already have on the mask, and all they have to do is turn on the oxygen.
To allow passengers to breathe sufficient oxygen, needed to survive, even in high altitudes.
I believe the FAI world altitude record for a helicopter is 40820', and helicopters can now land on the top of Mount Everest, just over 29,000'. But practically speaking, helicopters rarely go over 10,000 feet as that's the altitude above which pressurized cabins and supplemental oxygen are required.
Aircraft cabins are pressurized to maintain a safe and comfortable environment for passengers and crew at high altitudes. Pressurization helps prevent hypoxia (a condition caused by low oxygen levels), allows for normal breathing, and reduces the risk of ear and sinus discomfort during flight.
They could not remain conscious unless they either pressurized the cabin of the plane or wore a face mask that supplies them with pressurized air.
Outside the plane is way less pressure and less air to breath. If inside the plane was the same pressure as the outside, people might die depending on how high the plane is. ***************************************************************************** FAA law mandates an un-pressurized aircraft the pilot and co-pilot must wear oxygen masks at or above 12,000 feet ASL altitude. Hypoxia sets in with no warning and no symptoms.
Cabins of jet planes are pressurized to ensure a comfortable environment for passengers by maintaining a breathable atmosphere as the aircraft flies at high altitudes where the air is thinner. Pressurization helps prevent hypoxia, altitude sickness, and oxygen deprivation.
Most airplanes fly at high altitude because it is faster and safer as long as the cabin is pressurized. There is less clouds and turbulence. The airplane uses less fuel.