It's required for how revolver weapons operate. You cannot have the cylinder floating at all as even a few mm of difference can result in catastrophic failure. People saying revolvers are more "reliable" are full of fucking shit. The damn things are like clocks with their own set of springs and similar to operate.

It was true that early semi-autos were unreliable in keeping from going off suddenly with a round chamber and hammer back. That was early ones though, not today's. While it's not 100%, it's still much safer than a revolver with the hammer back.