Some may think...
Revolvers are in a normal "relaxed" state... No spring tension on the hammer... which is already forward and uncocked. Nothing can happen with the gun until the hammer is pulled back either by DA trigger or pulled back manually. Hard to imagine that happening unintentionally.
Semi-autos, on the other hand, when they have one in the chamber are in a cocked state. Racking the slide to chamber a round, cocks it. The hammer/striker is now spring-loaded, under tension and ready to slam into the firing pin and blast off a round. So one could imagine a part failure allowing the hammer to drop, or an accidental snag pulling a light SA trigger. They are just too "ready" to go blammo.
Some truth... some unfounded fear