Why do some people drink alcohol and take drugs?
Isn't it nice to have all of our needs met so that we can indulge in mood altering substances. We don't have to forage for food, find clean water, build our shelter, or worry about where the waste goes. Modern life is so plush that in order to occupy time, we have to ingest drugs/alcohol. People who partake in this exercise are the weak in society and not inclined to make the effort for real achievement. The people who get their high through individual achievement are the ones who create the world that allows other people to waste their life away. Our current state allows for the existence of both. The answer to your question is human nature--the bad part of it.
It's apart of our nature. Humans always have, and always will have people who will indulge. It's always been. Which is why the war on drugs was such a massive waste of time and resources. Drugs have never been easier to obtain and have never been cheaper. And thanks to the crackdowns, manufacturers have had to rely on scientific methods, rather than the ones previously used in nature, ultimately creating more potent drugs. I'm obviously not saying alcohol and drug abuse is okay, but it's something we as a society need to accept, because it's not going away. And we'd do more good by creating programs to help addicts, instead of incarcerating them. And creating safe, legal avenues to purchase drugs and limit their consumption, instead of letting drug dealers rob our economy blind.
They like it.