Why don't people believe that alcohol is a drug?

They aren't aware. Alcohol use is a widely accepted way to relax; it is used in many social contexts. People may put pressure on each other to drink, to be part of the "party." people don't tend to put it in the same category as other drugs because they're in denial about the harm it can lead to in some individuals. Although awareness is growing, many are equally blase about nicotine.