If drinking alcohol is so bad, why isn't it illegal?

They tried that ....called prohibition...led to the growth of organized crime. If drinking toxic substances is your thing do so in moderation. People are free to do all sorts of things that are self destructive. When you do so to the detriment others (drive drunk) they can toss you under the jail for vehicular homicide. Most do not go that far, but a lot kill off their livers.

This is a philosophical question of free will. You might want to read some Socrates and Plato and all those other Greeks. Not to mention Alexander Hamilton.