The problem with "I know it when I see it" definitions is that there's an inherent lack of consistency, in that two different judges aren't guaranteed to reach the same conclusion.

For instance: fair use of copyrighted content. The only way to know for sure whether something is fair use is to go through a lawsuit. You can be pretty confident in advance, but you genuinely can't rule out the possibility that the judge will disagree with you because of something entirely subjective.

Writing an objective technical definition can lead to rules lawyering, but the fix there is to improve the rules and close the loopholes. Otherwise you're always at the mercy of being sued and happening to draw a judge in your case who makes a weird decision.