If the intent of policy X is to achieve goal Y and X does not in fact achieve Y, what is the point of continuing to pay the costs of X instead of proposing some X' that actually does achieve Y? Continuing to advocate X when X doesn't achieve Y suggests that the proponents of X don't really care about Y, but instead some unstated Z.
Because a simple and easily enforced X that hits 90% of cases is vastly preferable to the 1200 page tome that makes up X’ and requires 175k people to enforce after 20 years of law writing.
In some situations, "perfect is the enemy of the good" is a reasonable approach. Unfortunately, we're not dealing with a good-but-imperfect filter, but a practically useless filter, one that imposes significant costs on top of being ineffective.
"Weapons of Math Destruction" presents good evidence that 90% is much lower than the actual effectiveness of X.
