The announcers have earphones on so they can hear the director/producer. How do you think they know what replay is being shown? They also have spotters telling them who carried the ball, what line the ball is at and other stats. Try talking about something when someone is talking in your ear.
I know I couldn't do it at my age. My hard drive is too old with limited capacity. It's amazing they don't make more mistakes. When an announcer calls the wrong name for the ball carrier, chances are it's what he was told to say.