I guess it depends what you mean by stupid, but there is plenty of science to back it up. The longer you've been driving, the better driver you are. The better driver you are, the better drunk driver you are. It fits with everything we understand about skills and automatization. Not to mention fatalities went way down after the law changed.

Driving is super complex. It's not just coordinating your feet and your hands. It's monitoring all the cars around you, judging speed and distance, reading signs and symbols, all while navigating a one ton box of metal at speed.

Meanwhile, none of the health benefits of alcohol outweighs the detriments in the end, and the earlier you start to drink, the higher your risk of alcohol. The UK, Europe as a whole, and Canada all have greater incidence of alcohol problems than the US. So, with no data-backed benefit to a lower drinking age, it hardly seems stupid to me to decide the drinking age based on a desire to reduce fatalities.