Governments hate anything that empowers individuals. This is because it makes governing harder. And this holds true regardless of whether you view government as overall good, or generally evil.

Say the U.S. government has the best intentions for making the drinking age 21. Surely some 18-year-olds could handle drinking, and surely many 25-year-olds cannot. But the government would have to expend massive resources evaluating the capacity of each individual to handle their alcohol.

Of course, this would also be extremely oppressive to allow a government to decide on an individual basis who has certain rights and can handle responsibilities. But it is no less oppressive to deny 22 million 18-20 year olds (still picking a relatively arbitrary age of responsibility) the right to decide what will go into their bodies.