The answer is simple when you are climbing and you are having trouble breathing is because you are in a higher elevation.
There is not much oxygen
A mountain goat can climb to however high it wants to climb. That's like asking you, how high can you climb?
When you climb a mountain it gets harder to breath because the higher you get the less amount of oxygen there is and the more pressure. Your lungs are not used to that so it gets harder to breath.
Atmospheric pressure decreases as you climb a mountain because there is less air above pushing down on you. This decrease in pressure can cause symptoms like shortness of breath and fatigue at high altitudes.
You get out of breath quicker because the air gets a lot thinner the higher you go. The highest point where there is no air at all is space.
To climb Mount Everest or any high altitude mountain you need two things, they are money and experience.
Air evaporates
Try Alaska
The air is thinner, there is less oxygen molecules per volume of air (the air is less dense) therefore to absorb the same amount of O2 into your blood at 4000ft as at 0ft altitude you have to breath more/ more deeply. Also, you're climbing a mountain (going uphill) which uses more energy than going flat as you have to work against gravity.
It is less.
3 feet People have climbed Everest.
the less oxygen