Because the small amount of surface agitation (where the oxygen exchanges) that air stones create isn't enough movement to create the oxygen levels required in SW
but is enough for FW
tanks. I still used power heads and strong circulation in my FW
tanks personally though even though it was over kill. I never liked having to clean the clogged air stones myself.
Also some deep water corals like sponges can die if exposed to air bubbles.
Water movement alone doesn't create the oxygen which is why it's important to aim your powerheads towards the surface to agitate the surface. If you have an overflow/sump then it's not an issue and you can place your output lower since the action of the water falling will oxygenate your water.
That's also why we recommend topless tanks to keep carbon dioxide from building up which will affect ph levels and keep oxygen from exchanging as well.