Why do famous musicians do drugs?
There are some musicians who have gained inspiration and/or popularity from being associated with one drug culture or another --for instance, there's a long list of musicians who are particularly popular among pot aficionados.
In my opinion, however, the main factor impacting celebrity drug abuse is money. When a person has a lot of money, they attract people looking to get their piece of it, and a drug addiction is one of the most reliable ways to drain someone's bank account.
There's also the fact that the celebrity's wealth makes it much easier for him or her to easily gain access to drugs that might be out of reach for an ordinary civilian.