Why does air pressure go on changing as the altiude changes?

Answer 1

Gas is subject to gravity in the same way as everything else. The Earth retains its atmosphere because of gravity. Gases have a general tendency to expand to fill all of the space available to them, but the gases that make the Earth's atmosphere cannot expand for ever into space because gravity causes them to stick to the Earth. The balance between expansion and gravity means that atmospheric pressure is higher where gravity is higher. As you move away from the Earth, gravity is weaker and the gases that make up the atmosphere can expand and so have lower pressure.

Answer 2

Air pressure decreases with increasing altitude because there is less atmosphere above, resulting in fewer air molecules exerting pressure down. This decrease is exponential, meaning the drop in pressure is more rapid at higher altitudes.