Why the plane is dry inside

Air pressure and humidity inside the plane Very low !!! The inside of the plane is 0.8 atm, Humidity is only 20%. Humidity is only 20%, almost desert-like. Why is it so low? Especially when taking off, the air pressure is very low, The phenomenon is probably something everyone has experienced. When taking off, it rises sharply, so the density of air decreases. It looks like On the contrary, when..