Why are gills not suitable for breathing air?

Gills are designed to absorb oxygen only from water, and can only work if water is passing through them or they will collapse. When a fish is taken out of the water, the oxygen-absorbing filaments of the gills flatten out and the fish suffocates because it's gills are not made to absorb oxygen directly from air.

The simple answer is that gills colapse when removed from water as they are adapted to water which is more dense than air.