The first thing you have to do is look at a guy like Artur Mirzakhanyan.
Go ahead, watch the video. It's hard not to sympathize for the poor guy. For starters, his opponent is a good deal larger than he is. A punch splays Mirzakhanyan across the mat early, despite it being a short short thrown off the back foot. Then you watch his opponent rip his arm free of a submission attempt, only to bring that same arm, maybe half a second later, down between his eyes.
It was over in less than a round. Mirzakhanyan was not the first guy to get his own backside fed to him by Petr Yan.