system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : why do the Blue Angels not wear any oxygen mask during flight? won’t they run against the risk of getting hypoxia?
gpt
In airplanes, either you wear an oxygen mask or you have a pressurized cabin in order to ensure the appropriate concentration of oxygen present when you inhale.... this is above 10,000 ft. Below 10,000 ft or so you can breathe normally. Above it, you need an oxygen supply or artificially pressurized air. 

In a fighter jet, the cockpit is pressurized to a certain degree, but not to the point where it would suffice at all altitudes the airplane can fly in - the pressure differentials involved would require a lot of extra reinforcement which would make the jet heavier which would make it less maneuverable. So the cockpit is partially pressurized but the pilot still has to wear oxygen above certain altitudes. Plus its probably just a good routine for the pilots to wear all the time just to be safe. Commercial airliners have entirely different design goals in mind; their design specifically accommodates the need to pressurize the entire cabin to a breathable altitude... but neither does it need to hold together in a negative 7g dive, for when you keep up foreign relations.

For demonstration teams like the Blue Angels however, all of their practice and demonstration flights take place well below the 10,000 ft altitude they need oxygen at. They might still wear oxygen masks when flying cross country, but for shows etc. a mask hanging there is just another thing to get in the way/be a distraction when they really have a lot of other things to be concentrating on.