I think it is rather a cultural thing and also mixed with the regulation which each country has regarding the advertisment of products and services which are intended just to be consumed by adults. For example, we don't see billboards advertising pornography websites by the roads and streets of our cities, because the perception of the public is that children and teens would have easy access to those services with little restrictions.
In the case of alcohol, it is a good which cannot be acquired by minors so easily, since they would need to show a ID card so society are more comfortable to show billboards of alcoholic beverages in plain view.

In the case of gambling in different, because most casinos do not ask for documents to sign up and deposit money in order to play their games, they only ask for KYC documents at the moment of withdrawing money or in specific occasions when the staff believe it is necessary to do so. So gambling has some similarities with pornography when comes to its easy access by minors and teens, since we are talking about services which do not ask KYC documents immediately to access to them.

Casinos just ask a email address, verify such address and you are ready to deposit and get on it.