I'm guessing for safety of the returner. If he muffs a punt the first thing that's going to happen is he'll get blown up. If it is immediately blown dead then there's less reason to hit the returner.
I think because it's technically never in the possession of the returner. Pretty sure a kicking team cannot advance a kickoff either. If he catches it, establishes possession (like a catch), then fumbles, it's a different story. One of those weird rules like why spiking the ball is not intentional grounding.
Because there's an overwhelming probability that the punting team will be waiting there with numbers to recover it and likely return for a touchdown. Pretty cheap TD
Possession has to change. You're giving up the ball by kicking it so the opposing team has to establish possession of the ball before it can be advanced.