Because evolution doesn't work to produce good solutions, just good enough ones. For example, your body works out it is in a low oxygen environment by sensing a

Because evolution doesn't work to produce good solutions, just good enough ones. For example, your body works out it is in a low oxygen environment by sensing an elevated amount of carbon dioxide, not measuring oxygen levels. As such, it feels painful when you hold your breath, but going into a room with only nitrogen won't make you feel anything, except maybe breathe faster.