MikeT,
Glad you asked. While I realize that my reason is quite argumentitive, it is simply maturity, particulary in regards to our sex (male). I don't know your age, I'm 54. The older I get the more time I realized I had when I was younger. I didn't need to be spending it in a bar. I am fairly liberal and I don't force my ways on anyone. I raised two sons and I allowed them to drink in my presence, not their friends and not away from the house. As a result I ended up with two sons who are responsible adults with families. They never spent time in bars and still don't.
I don't think 18 year olds make good decisions typically without help from adults. You put them in adult situations without guidance and get them looped and they'll get hurt or hurt someone else.
Drinking is for mature adults. 21 may not be that number for some people either, but it's better than 18. Remember, I was 18. too.