As they do the ability to regulate interstate commerce of alcohol. Or tobacco. Or carrots. Which is why the 18th amendment was pretty useless, given how much alcohol is transferred interstate.
The 18th was desired not required. The reformers thought it would be more permanent than a simple federal statute.
About half the states had already banned alcohol prior to the 18th, but the states lacked the power to stop interstate shipments from "wet" states to "dry" states. Congress passed the Webb-Kenyon Act prohibiting this interstate commerce, but it was ineffective.
The only solution was a nationwide ban which, it turns out, was unpopular. I bring this up to point out that turning the drug legalization decision over to the states would be equaly fruitless.