that's an easy one.
Because if everything that could be detrimental to society, or even a single individual was illegal, you wouldn't be able to do any of the following:
drive a car
eat anything with saturated fat
drink alcohol
have sex
post on the internet (carpal tunnel is a bitch)
cough or sneeze in public
etc etc.
Is that the kind of place you wish to live in? We all make personal choices as to risk/reward. I'd say we all engage in some sort of risky behavior (some more than some). Those are choices I choose to leave to the individual as long as it can stay more or less confined to the invidual.
Alcohol users put non alcohol users at risk every single day. I agree with the legislation and laws we have around it. We persecute drunk drivers vehemently. In California its a FELONY to drive intoxicated, the legal limits are VERY low, and your 2nd offense is gonna get you mandatory jail time! I have no problem whatsoever with this because driving drunk is not a simple personal choice, its a choice to put other's in harms way.
We don't attack the alcohol use, you can drink in the privacy of your own home as much as you want, but we attack the negative effects and behaviors that could result from it. Its not illegal to be an alcoholic, and we'll still even treat you for liver disease and the like that in some cases cost other people indirectly.