In the USA, alcohol laws are very strict and specific, while in most other parts of the world they are very lax. What's the reason?
The "frame challenge" aspect of this answer is to note that, while the USA is stricter than many European countries, it is less strict than many other countries. Many majority Muslim countries ban alcohol outright, or put far greater limits on its sale than the USA.
However drinking culture is more restricted in the USA than in Europe. The age at which you can purchase alcohol is only a little higher in the USA, but there is a difference. Much of this can be traced to the consequences of Prohibition, and the culture in which Prohibition arose.
There is a religious tradition of Protestant piety in the USA, in which personal behaviour is emphasised over ritual. This led to movements that opposed alcohol absolutely in the 19th century and culminated in the constitutional banning of all social alcohol in 1920.
Even after repeal of these laws in the 1930s, the culture of alchol consumption remained very different. The cultural effect of prohibition extends to the present day.
The higher minimum legal age for alcohol use in the United States is tied directly into the low minimum driving ages. The U.S. has the largest population of the Western World, and is fairly rural outside of the coastal regions (even then, some coastal states are not that densely populated). This creates the need for lower minimum driving ages in these states. Typically the minimum driving age in these states is 16 although some states allow for hardship license which allow for people as young as 14 to drive, usually in very rural states and usually only if a legal adult guardian is unable to drive; most of the other countries in the Western World don't issue drivers' licenses until 17-18.
Drivers who are the minimum age of driving in any nation have the highest insurance premiums for a reason: young and inexperienced drivers are some of the worst drivers. Since the driving age in the United States is low, a high alcohol legal age is seen as a trade off of sorts.
In 1984, the United States Congress passed the National Minimum Drinking Age Act, which withheld 10% of federal highway funding from states that did not maintain a minimum legal drinking age of 21. As a result, the current legal minimum is set as a condition of Interstate Highway Funding (roads in the U.S. are usually funded by the states, but the Federal Government funds interstates because they were considered defense infrastructure that was vital to mobilizing troops in an invasion of the U.S. or a disaster). This gets around the state issue by tying payment to states restricting alcohol sales to 21 years or older. Since every state has roads that are funded by the Interstate System the states all upped the age to 21 to get their road funding.
Perhaps the fact that 10% of American adults waste their lives in a state of perpetual inebriation, leading to a corresponding loss in the GDP and fracturing many families, destroying life, and contributing also to a conservatively estimated quarter of a trillion dollars in additional losses annually, might help to contextualize the USA's relatively conservative stance and strictness about alcohol.
Those who claim that alcohol is not addictive or who promote the punchline "drink responsibly" are probably dodging the fact (derived from the Washington Post source above) that at least 88% of alcoholic beverages in this country are consumed by alcoholics.
It's addiction industry at its finest, and US citizens are unquestionably wise insofar as they restrict and curtail it.
Relative to most of Europe, U.S. consumption appears to be below average but not extremely low (9.2 liters per capita annually). It would be interesting to see a breakdown similar to the Washington Post article for other countries.