Unwritten rules are unwritten because they are not worth writing down.
Imagine when they wrote the rulebook…
“I’ve got this idea called the Infield Fly, say there are runners on first and second, maybe third as well, and… [3 hours later]” “Yes, we definitely want to write that one down.”
“And me, I’ve got one called a Balk, where we make the pitcher step off before he throws over…” “Why? OK, let’s just write it down.”
“And my idea is that the batter can’t bunt to break up a no-hitter.” “No, YOU are just being silly, we’re not writing that one down.”
“OK, I’ve got another, when the batter hits a home run, he’s not allowed to look at the ball…” “No, I think we’re done here. Let’s just print the thing.”