Because the design of the AR is pretty much the complete opposite of what you need to make a piston gun. You see increased wear on things like locking lugs and camming reliefs in receivers because you're removing important steps like the gas return through the tube forcing the bolt to cam against the breach face instead of just ripping it out of the camber, bashing blot lugs against chamber lugs.

They're just bad ideas with no real merit to them, and arguments in favour of them are people that own then or people that don't know anything about the engineering of the AR-15. You want a piston gun get a piston gun like a SCAR. You want a Stoner Gas gun, get a Stoner Gas gun.