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Most of us have probably flown in airplanes on altitudes greater than that in pressurized passenger airplanes and didn't feel any fatigue at all.
I think you gave a partial answer to your own question here. The plane is pressurized. The cabin pressure in a passenger jet is only equivalent to about 2400 m of altitude.
Likewise climbers can bring their own oxygen with them, but still complain of incredible fatigue.
The earth's atmosphere is about 1/5 oxygen, and the pressure at the top of Everest is about 1/3 the pressure at sea level. So theoretically, if you breathe pure oxygen at that altitude, you could have a partial pressure of oxygen that's higher than at sea level by a factor of 5/3.
So why can't supplemental oxygen make you feel just fine at 8000 m?
Others may be able to give more authoritative answers, but I think the general idea is something like this. Your body has membranes in the lungs and brain, and normally various fluids and gases are transported in both directions across these membranes. Your body also has CO2 sensors that are what tell you how much to breathe, and these processes also maintain your blood's pH at some correct level. All of these physiological mechanisms get messed up more and more as you go up in altitude, and simply bringing the partial pressure of oxygen back up doesn't fix everything. To some extent your body can adapt, but this takes months, and there are limits to the possible adaptation. As a result of all this, the level of oxygen in your blood is still too low. (There are also going to be various issues such as dehydration, malnutrition, and sleep deprivation, none of which can really be completely eliminated, and all of which will cause people to feel exhausted.) This causes a gradual deterioration of your health, which is only reversed by descending -- hopefully after you reach the summit that was your goal!
I think there may also be practical limits on the use of supplemental oxygen. I don't think a climbing party can always carry enough oxygen so that they can open up the regulators to full blast for every climber for 24 hours a day. I think it may also be difficult to simultaneously sleep properly and breathe properly while breathing through an oxygen mask.