Why do gas bubbles flatten the surface of a sponge cake after being left out for a while?

Gas bubbles in a sponge cake provide structure and lightness during baking. However, when the cake is left out for a while, the gas bubbles can escape or collapse due to the cake's moisture evaporating, leading to a loss of volume. This results in the flattening of the surface as the cake loses its aerated structure, causing it to become denser and less fluffy. Additionally, exposure to air can dry out the cake, further contributing to its flat appearance.