Alcohol has been used since before the beginning of human history. Attempts to stop its use have been carried out from time to time in various countries, and in some countries it is not used for religious reasons. However, attempts to prevent its use for non-medical reasons in our society have failed.
Almost every society has some psychoactive drug that is tolerated, that is incorporated into its traditions, its practices. Alcohol has been in our society for a long, long time. If we were starting from scratch, it's conceivable that we might have picked cannabis instead of alcohol, but we're not starting from scratch. When we talk about cannabis, we have to think about whether we are or are not adding cannabis to alcohol. That is a consideration that means that, necessarily, we are treating the two drugs somewhat differently.
Certainly you are right that alcohol also causes problems and is tolerated. Cannabis causes problems, and currently is not tolerated. It's illegal. When a drug is legal, the use tends to increase greatly because of easier availability. The price is usually cheaper when it is from legitimate sources than from illegitimate sources.
Those factors tend to increase use when it is legal. That is another consideration that has to be taken into account when doing cost-benefit balance. It's not just what the balance is now, but what the balance will be if we change practices in a way that influences the extent of use.