If a fish breathes oxygen through water, why can't it breathe out of water?

Question

why can't a fish breathe oxygen out of water if it just breathes it from water?

Answer 1

Because fish do not have lungs(apart from the rare acception of the lung fish). they have evolved in such a way where water is required to flow over the gills for it's breathing apparatus to function properly. It's like saying if there's oxygen in water, why can we not breath water, different physiology and anatomy deems it impossible.

Answer 2

Fish gills are remarkable things, but the conditions under which they function are pretty specific. For one thing, they are rather delicate, and their tremendous surface area (the main thing that makes them work so well) is dependent on being immersed in water to support their weight. Out of water, the gills collapse like wet tissue paper, and very little surface area is left exposed for gas exchange. Most fish, therefore, can only survive a short time out of water before oxygen deficiency catches up with them and they asphyxiate.