everyone misses a layup every now and again and you dont always know you are as wide open as you are (so the belief that a defender is close to you can put you off). Also if you get fouled a few times or in a big way it can knock you off your game so that you miss the shots youd normally make.
taking these aspects of real life basketball into buzzerbeater means that every now and again players will miss open layups. good simulation :)