The point of the rule isn't enforcement, it's setting standards for good-faith contributors.
Your assumption that all rules must be about enforcement is incorrect. Your assumption that only that which can be measured matters is incorrect. I don't know where this belief system comes from, but it strikes me as profoundly toxic.
By this logic, we obviously shouldn't ban drinking and driving - there's no way to test every driver every time, and presumably those most skilled at drunk driving would be undetectable, so it's really just religious moralism.
"Good drivers don't drink and drive even if they think they can get away with it" is just a no-true-scotsman argument, and thus we should actually encourage people to drink and drive so that they get better at it. Nobody should ever have any standards that can't be automatically enforced by a linter, after all.
And look: https://news.ycombinator.com/item?id=47340079
Unenforceable rules might just be the backbone of society, if you think about it.