You lose oxygen intake at sea level just wearing a face muzzle, once at altitude the cabin is pressurized to between 5,000 and 7,000 feet, so in effect wearing a mask while flying is exposing most people to mild hypoxia.
And I used to live at Lake Tahoe, its near 6,224 feet and I worked up at Heavenly the ski resort at 8,000 feet and I know how it changes people, they get irritable, they fight, they cannot make proper decisions.
Thats exactly what masking is doing.