They're not being affected by drug use, they are being affected by the environment in which some drugs are distributed.
The profit margins are so enormous when it comes to selling illicit substances that it's no surprise why people continue to sell them. And it's not like you can go to court to settle disagreements between rival drug sellers, unlike with tobacco, alcohol, or pharmaceuticals. There is no legal recourse so the only solution is by handling it yourself and cartels are just well funded gangs so they'll have no problem taking care of business on their own.
People don't gun each other down over alcohol distribution and people don't hang other people from bridges to gain the anti-depression pill marketshare