Drinking....!

Its crazy because in London the age limit for drinking is 18. Although you can drink in pubs with your parents at the age of 16. So since I lived in England until the age of 20 - I was used to drinking with my friends, and we regularly went to pubs etc.
Imagine my surprise when I moved to America and found out that the age limit to drink was 21. It wasn't that drinking was that important to me, but still. It was bizarre. And suddenly I couldn't drink, even if I was out at a restaurant with my parents. And I couldn't go to bars with my friends (although my friends were the same age or younger, and there were no bars in Florida lol). But it wasn't too long before I did turn 21 - and of course I still get carded but whatever. Its not that i necessary want to drink, its just that after being able to drink in the UK and having that freedom, suddenly all that was taken away. Isn't it weird that the age limit for drinking is 3 years different in 2 different countries? Why is that?