Re: Why oxygen generators?
Well it depends on how they supply the pilots oxygen.
If it's a closed loop system then that will mean the gas they breathe out is cycled back through the loop, scrubbed of CO2 and analysed. If there's a requirement to top up the O2, then the required amount to keep the gas breathable will be added.
If it's not a closed loop, then the gas they breathe out is just vented away but I'd suspect it's not open as this would mean over 90% of the oxygen breathed in by the pilot would be wasted on exhale. Plus it's presumably closed due to things like the cockpit being sealed.
A closed loop also has the advantages that a) that 90+% of O2 isn't wasted - it's recycled round the loop and b) any O2 tanks will be smaller than otherwise needed.
I am coming at this from the perspective of a hypoxic trained closed circuit rebreather technical diver. With a pair of 3l cylinders (one for O2 and one for a diluent mix) I could potentially get around 6 hours underwater - I say potentially because various things alter that such as how long the scrubber material can last when removing the CO2, how warm or cold the water is etc etc. But compare that to diving on "open circuit" with 2 x 12l cylinders - they'd tend to last me for an hour of diving.
Also even as a diver you are trained to spot hypoxia, CO and CO2 buildup, Nitrogen Narcosis etc etc so I'm amazed that these pilots aren't being thoroughly schooled in this!
Oh and just an edit: the rate at which an individual metabolises O2 doesn't change between any changes in atmospheric pressure so no matter how high you climb or how deep you go, you will still burn O2 at the same rate.