They don't fly out of regional airports at least in NYC I think that is rare.
I"ll bet there are a lot of operational issues.
Delta, United and others own low-cost carriers (the famous United case where the senior was dragged off the plane screaming and kicking was not on United but a low cost fully owned subsidiary carrier of United) as it is, so they could have a low-cost carrier that travels to Europe. These low-cost wholly owned carriers are domestic only.
But none of them have them because they want to cannibalize the higher fares charged. They enjoy their margins.
Norwegian (like Amazon, like Uber) is in a contest to capture market share and thus may be willing to operate on thin margins or even take a short term loss to capture share. The major American carriers may not want to invest the money to retain share and prefer to use politics instead.