Feeling sorry for someone means you empathize with their plight. Especially if it's a family member. Empathizing with an addict is different than enabling them to continue to hurt themselves.
Why do many people feel sorry for drug addicted people?
I can feel sympathy, even distress on their behalf but, as your question implies, they did it to themselves.
The same applies to people addicted to anything. In the beginning it's self-inflicted but after a while there's little choice. The problem is that the purveyors of drugs, booze, cigarettes, and so on, get to them when they're still kids -- especially when they're teens and will do anything stupid just to give their parents the finger.
My sympathy doesn't extend to the point where I condone them breaking the law or sponging off their family and friends.
I don't know. They do it to themselves.
Because a drug addict is going through difficult times. Many of them find it hard to live anymore and the only thing they live for is the drugs. I'm addicted to tobacco and trust me, getting over an addiction is not something that's easy. It's difficult.