Same reason you can buy a shotgun at 18 but must wait till 21 to buy a hand gun. Some states just believe that they are not equipped to safely and responsibly handle it. Fun, has nothing to do with it. Why do you have to be 25 to rent a car? I know it's not a law, but it is a standard.
Ah, so it doesn't make sense, then. Gotcha.
Seriously, this is another matter for the states to decide. I have no problem with some states/counties/cities choosing to disallow the purchase of alcohol on Sundays because it's not a federal law, and those who live in areas that have such laws know that they can move elsewhere if the option of drinking on Sunday is that important to them. I'd feel the same about some states not allowing the purchase of alcohol by a legal adult until the age of 21. It's the fact that the law is federal that makes no sense.
Good example, especially since the states have no business telling anyone what guns he can buy.
Why do you have to be 25 to rent a car? I know it's not a law, but it is a standard.
It can be an industry standard all you want, but it's not comparable to a law. Why? Because the car companies own their cars. They can, or ought to be able to, refuse to rent to anyone for any reason or no reason. That's because they have property rights. An "underage" drinker also has property rights.