Sorry for such a late answer, but I wasn't even going to answer until I saw this scene when I had the TV on in the background while I was working. I think it pretty much wipes out a lot of the speculation.
They live in the sky by choice, either because they need to leave room for factories or parks, or for other reasons, but now that we have two screen shots showing that the ground is a pleasant environment and one they visit and even play in regularly, there's no indication anything keeps them from living on the ground.
They do show buildings on the ground, in at least one case. In Private Property, the episode where Cogswell builds a building next to Spacely's, note this scene toward the end. The building inspector measures the building height, and there is a shot of the end of the tape measure hitting the ground.
Here's a screen shot:
(From Private Property)
You can see this shows what seems to be a metal sidewalk (judging from the sound of the weight at the end of the tape measure hitting it). There's a yellow building in the background, with bushes and trees around it that doesn't look damaged. The curved beams (of metal or whatever they use) would be supporting Cogswell's building. The red circular pillar probably supports Spacely's buildling and you can see another elevated building in the background.
There's no indication of pollution or that the surface is not used. This indicates that it's more likely that with gravity belts and other technology that can make heights safe, they live in the air because they choose to.
(From Millionaire Astro)
This one also shows a livable and pleasant area on the ground, so it's pretty clear there isn't an issue with pollution or anything similar.
It's because if enough buildings are up high, it leaves more room for factories or other buildings down low and lets them use a lot of the ground area for parks. But that, of course, is speculation.
What this does show is that the surface of the Earth is not dangerous or that there's a need to avoid it. That makes a strong case for them living in the sky by choice, rather than need.
I'd like to add (much later, after a couple months) that I often have the network that is rerunning The Jetsons in the U.S. playing on my TV in the background while I'm working. I've seen even more episodes that show the ground in screenshots and they all are similar to the shots I've posted. If, for some reason, more screenshots are needed, I can capture them when the episodes air, but I think this makes the point as it is.