Firstly, this is decently LY.
A decent way to think about it is to think of a plastic balloon vs shopping bag. Inflating a balloon, and leaving an opening will force the air to escape as the elastic will return to the non stretched smaller shape. Inflating a shopping bag will not result in the shopping bag wanting to return to the normal state, and it will most likely stay inflated, as there is no force that wants the shopping bag to return to an not inflated state. Lungs of course inflate by a different mechanism, but the elasticity of the lungs is part of the mechanism that drives deflation as you exhale.