A lot of the time, people ignore over small, simple things because they either assume someone else already took care of it or that it's such an obvious thing that how could anyone not notice. The checklist forces you to actually look up and see that the extra blood needed for the surgery didn't show up or that one of the surgeons present is actually supposed to be in another OR.
It's like help desk asking you if it's plugged in. You may think of yourself as a technical person and that the problem is more complex like the driver isn't loaded or the monitor isn't properly configured but the problem could just be that you forgot to plug the video cable in. Just having those basic checks occur before anything can go wrong makes it so that when things do go wrong, time doesn't need to be wasted on going over the simple stuff or even worse, forgetting about the simple stuff and going down the wrong route.
It's like help desk asking you if it's plugged in. You may think of yourself as a technical person and that the problem is more complex like the driver isn't loaded or the monitor isn't properly configured but the problem could just be that you forgot to plug the video cable in. Just having those basic checks occur before anything can go wrong makes it so that when things do go wrong, time doesn't need to be wasted on going over the simple stuff or even worse, forgetting about the simple stuff and going down the wrong route.