Why is it just alcohol and gambling and not, say, cat food and toothbrushes? I understand you don't like alcohol and gambling, but how do you derive that others should be prevented from seeing what you don't like?
It is not a good idea to force an ad for beer on an ex-alcoholic/someone trying to stop alcohol, for example.
Things such as gambling, smoking and (though somewhat debate-ably) alcohol all fall into this category.
Regardless of whether you agree with it or not, many countries now ban the advertising of cigarettes, so realistically, it's not too alien of an idea.
Allowing users some granularity over ads makes sense for everyone. I am never going to buy nor rent a car so showing me car ads is stupid. Allow me to opt out of seeing car ads and the advertisers save money and the content I'm watchig doesn't get negative associations and I am less likely to install ad blocking software.
But if I create a video and host it on YouTube I would want to prevent adverts for gambling or alcohol to appear next to that video. I'm not sure this is offerred by Google.