As an umpire, I know from experience that most of us know when we get a call wrong, and most "blown calls" we know we blew. This isn't the case all the time. But it's not like we can just change our minds. Instant replay lets the wrong be righted when we know we make a mistake.
I love the game, and I love being on the field as an umpire since I couldn't get to play beyond HS, but being behind the plate is by far the best position as an umpire. You're excited for your plate games all day. Not being able to call balls and strikes would take most of the fun away, at least for me.