We've seen gateways raised slightly above grounds late in the game. Logain managed to do that to his (which means he found out how to alter his weave to make this happen to his gateways), to make sure he wouldn't destroy anything on the Sea Folk ship he travelled to. This impressed the windfinder whose POV we saw that scene from. So I would say "it has not occured to them yet" is definitely one of the factors involved. Aerial warfare is a new concept in the Westlands. It's not likely something the channelers have given much thought to at this point.

I guess opening a gateway in mid-air isn't impossible, but it's most likely not easy either, conceptually speaking. It might not be so easy to open it at a precise altitude too (you need to know the destination very well (have opened a gateway from there previously is best) or be in visual range to be extra precise with the point your gateway opens at, it's not for no reason channelers take no chance and so often open gateways km away from their real destination, in areas they are sure are empty of people, and why they've adopted the Travelling ground practice, points from which they've already Travelled, and thus know the exact weave to make to open a gateway to that exact point later.

It's difficult to say, because we haven't got much information on how exactly the channelers conceptualize the two points and how they translate that into their weave. We've also only seen so far gateways that open at 90 degrees to the ground. Those opened in mid-air would make it difficult/dangerous for the channeler to bend over to see below on the other side, without touching the razor sharp bottom edge of the gateway, and so far, we haven't see any gateway that's aligned at 90 degrees at the opening end, but aligned at a different angle to the ground at the exit end. It may or may not be possible to do this with a gateway.

The real advantages of opening gateways in the air would be extremely limited anyway. It would be much easier to achieve much the same results with gateways at the ground level: open a gateway, assess the scene and aim, throw a destructive weave at the enemy, close gateway. Repeat, but by opening the gateway at a different location in the battle, not leaving enough time for the enemy to spot you and aim back (you can also work in pairs, with one channeler opening the gateway and in charge of watching for incoming weaves and be ready to close the gateway, and another being "the weapon". If the gateways are made with saidin, the Seanchan can only detect them visually. Of course, with channelers able to read residue or copy weaves they see, that's really dangerous as the origin point of your gateways can be figured out and turn you into a sitting duck.

Demandred and others used a tactic like this at the Cleansing (but their opening gateways were spotted by the detection ter'angreal that can spot even inverted/reversed) weaves). Another thing is, we haven't really seen AOL-style massive scale fighting yet, except with Rand's inner circle at the Tairen manor, destroying an army of 100,000 in minutes. With weaves like these and small mixed gender circles using them, the Seanchan would be shredded in no time. Of course, a point remains that Rand doesn't intend to reduce the Seanchan to shred. His aim (except briefly in TGS) has always been to force them to his side, not to destroy them.