Why is a normal kickoff not a live ball until touched by the receiving team, but an onside kick is live?

An onside kick is when the ball is kicked at least 10 yards...and then it's live. The receiving team does not need to touch it for it to be live. The kicking team can recover it as soon as it travels 10 yards. But if the ball is kicked deep...it's not live until the reveiving team touches it. So how far does a kickoff need to be kicked before it's no longer a live ball? What am I missing here?