Huh, interesting. I guess this makes sense - to not take up a massive volume, breathing oxygen would need to be stored in liquid form, which means it would need to be kept exceptionally cold or under huge pressures (or both), and would inevitably boil off and need regular servicing. A chemical reaction seems to be a far lower maintenance (and thus likely much cheaper) option for devices that will hopefully never even need to be used during the service life of an aircraft.