The reason for that is; you know that not only 18 year olds drink, but already 15 year olds are drinking alcohol heavily, and that is the main problem. Raising the legal drinking age would only offend the almost adult teenagers (18 year old teenagers).
Younger teens are always going to find a way to get alcohol in smaller shops, where the merchant only wants to get some money. If not so, an older friend can buy some alcohol for these kids, maybe brother, sister or even parents. However, there are some good reasons why the drinking age should be raised.
This raise would probably aggrieve the not so heavy drinkers and make them not drink as much as they did before. It would lower the access for younger people to alcohol. If the government raises the legal drinking age, it sure will be a good start of fighting against drunk youth. I think that not all the teenagers are a problem, but only some of them, who don’t know the limits. Most likely there are many responsible teenagers and the drinking age raise wouldn’t be fair to these people and the already adult ( 18 year olds ) teenagers.