Skiers don't do crazy off axis rotations and grabs in the aerials because that isn't how they are judged. If you put one of those into your run the judges wouldn't know how to judge it so you would lose. Just like with gymnastics or figure skating. Every now and then a new move will come in but it is about performing known moves perfectly. Innovation is frowned upon.
For the long jump. It's not really a jump. The take off is flat and the ground drops away. The skiers are basicly gliding in the air on their skis. I doubt a snowboarder could get anywhere near it at the risk of screwing up the grooming of it. Would be cool to see though.