Guess what? You can! Circulate some air onto a rock, and if you're a really strong airbender like Aang, you can bend earth! Capture water with some air, and if you're a really strong Airbender like Aang, you can bend water! Now fire is a kind of different story.
But, it doesn't really matter! I don't know why the creators didn't realize this early. The battles could have been much easier before "The Blind Bandit" because we know Aang could levitate rocks with just airbending! The only thing Aang can't control is the freezing of water, and how earthbenders can just elevate their ground.
"Imprisoned" is a pretty good example of how Aang could do this. In this awesome episode, Aang blew some air into a vent and the air went through the other vent covered by a rock, and the science of pneumatics suggests that air can be used to push things in a pressurised tube!
