Except that you can't do that either because someone acting in bad faith can just say that there were other confounders, because there will always be other confounders, since we can't make a proper control group out of a society. Immigration/emigration, other policy changes on county/state/federal level, delayed effects of 30-year old policies (such as Roe vs. Wade or banning lead in gasoline), etc. There's no such thing as fair comparisons in the social sciences; we can only and have to act on imperfect information.