Here's why:

* The Atmosphere: The Earth's atmosphere extends far beyond the clouds. Clouds are formed in the troposphere, the lowest layer of the atmosphere.

* Oxygen Distribution: Oxygen is a major component of the atmosphere and is present throughout its various layers. It gradually thins out as altitude increases but is still present even in the upper layers like the stratosphere and mesosphere.

* Breathing: While the amount of oxygen decreases with altitude, there's still enough for humans to breathe up to a certain point. Commercial airplanes fly above the clouds, and their cabins are pressurized to provide breathable air.

However, it's important to note:

* Oxygen levels decrease with altitude: The air gets thinner as you go higher, meaning there's less oxygen available. This is why mountain climbers need supplemental oxygen at high altitudes.

* Different Atmospheric Layers: The composition of the atmosphere changes with altitude. The stratosphere, for example, contains a high concentration of ozone, which is essential for absorbing harmful ultraviolet radiation.