Exactly this....took me a while to wrap my head around this. The percentage of oxygen in air does NOT change with altitude, but the amount does. Air at sea level has the same percentage of oxygen at 20k feet, 30k feet etc. However air pressure drops with altitude, as do the absolute numbers of molecules on nitrogen, oxygen, argon, etc. The concept of "partial pressure" explains this more scientifically but is not terribly intuitive. There are two ways to fix this: increase the inspired fraction of oxygen above 21%, thereby making more molecules of oxygen available, OR force air at higher pressure into the lungs making the pressure (and amount) of oxygen more like breathing at sea level. The first method can help up to a certain altitude and is what happens with the loosely fitting masks that drop down for passengers. Certainly OK for a short period, say to go from 30K feet down to 10K. For the pilots it makes more sense to have tightly fitted masks connected to pressurized air (or air enriched with additional O2) of unlimited supply. That way they can maintain near normal intake of oxygen while flying the plane. Interestingly enough climbers in the Himalayas suffering from altitude sickness can be supported by pressurizing them in a Gamov bag: basically a foot pressurized habitat that is pumped full of ambient air so that the patient receives 21% O2 at higher than ambient pressure.