Air pressure is less at higher altitudes. Less air is available at high locations , so the air pressure in the atmosphere is also less.
The reason for air to be less dense at high altitudes could be due to the attraction of air (oxygen, nitrogen, hydrogen, water vapour and other contents) by the gravitation field of Earth.
Since the air density is less, the amount of oxygen contained in breathed air is less. Further oxygen being heavier among other elements in air, perhaps oxygen tends to stay lower on Earth.