Italian dubbing is really good mainly because we have a long, serious tradition in this field, with studios and professionals who've passed the craft down for decades! It's not like actors are born already good at dubbing, but there's actual formal training - lots of them take specific courses and learn how to sync their voice with lip movements, manage their breathing, modulate to convey emotions without the support of a physical body like in theater. Then obviously there's experience too, because when you work a lot in dubbing you develop a trained ear and an ability to adapt that lets you make any line sound natural, even when the original is poorly done!