Many of these DBS aren't smart. Watching Revis sort of spoils a niqqa into seeing how to play the position right. When these dudes have a receiver pressed to them along the sideline, they still face the receiver rather than keeping them on their back and looking back at the qb knowing that the sideline is their friend and that the only way a receiver could come back to the ball is going through you.
Other times, dudes bail out too soon and honestly, many of these guys aren't athletic enough to play the position to go along without thinking out there. To be a db, you have to be qb, knowing the down and distance, where you are on the field and who you're defending.
A lot of dudes go into playing db reacting rather than thinking. Asante Samuels may not be the greatest one on one cover man, but there spare some decisions that he makes on the field that is pure brilliance.