The amount of alcohol in any given drink is a known quantity. A designated driver is made available so that drinkers won't drive while drunk. We understand the effects of alcohol and we encourage people to drink responsibly. We have programs and clinics for those addicted to alcohol. There is nothing analogous to this for illicit drug users.
Street drugs are notoriously unpredictable.
The quality and quantity of active ingredients is generally a crap-shoot. Side-effects are often caused by additives that a drug dealer might use to stretch his profits. The additives could be anything from baking soda to rat poison. Unscrupulous drug dealers abound and they leave illness and death in their wake. In an environment of illicit activity, the safeties are off.