Yes, there is an unwritten rule. The rule is that there IS NO position player who can kick a ball 75 yards.
Do you know how hard that is to do?
I remember in the late 70's early 80's, when the Cards kicker got hurt (Neil O'Donoghue?), and the Cards decided to use LB Mark Arneson to kick off. This was a choice BEFORE they played the game. Instead of dropping a few bucks on a guy who could actually kick, they went with Mark Arneson! What a friggin' disaster, he was kicking ground balls. The "good" news is, they hardly put any points on the board that day, as usual, so there weren't too many kickoffs. Typical Bill Bidwillian move.
I am so thankful for Michael Bidwill. So thankful.