In the US, alcohol advertising is regulated by state and federal guidelines, as well as industry standards for self-regulation. For example, the Distilled Spirits Council has a set of guidelines for responsible digital marketing. Here's what they say about age verification:
Age affirmation is a process or a mechanism by which users provide their full
date of birth (month, day and year) and, when appropriate, country of residence
to affirm they are of legal purchase age. Age affirmation mechanisms may vary
depending upon available technology and examples could include, among other
things, an age affirmation page, an email or instant messaging age affirmation, or the use of a site’s “registered user” database of users of legal purchase age.
Why this standard and not an "easier" method? If I had to speculate, it would be so that all advertisers would have a clear, obvious method for verifying age that would both meet legal standards and prevent less-responsible advertisers from trying to game the system.
If the age verification process were not crystal clear, and an underage drinker injured himself or others after visiting an alcohol company's website, that could open the advertiser up to legal liability. With a form that the user has to complete and submit, then the alcohol advertiser could argue that they have done their due diligence and met their legal and ethical responsibilities.