Yup - gaseous oxygen isn’t very dense so you’d need pretty large tanks, and liquid oxygen needs high pressure and very low temperatures to stay liquid - neither of which is terribly practical for an aircraft, and especially for safety systems that need to be super reliable without lots of maintenance
I went through the US Navy’s damage control and fire-fighting training back in '81 and we trained with their oxygen breathing apparatus (OBA). I remember we’d have to pull a pin like in a grenade to light them off, and when we were done we’d have to release the canister into a barrel of water because they were like super-hazardous…
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.