Complicated issue.
People suck in general, so it isn't like they could actually go to someone and ask for help - without being severely and unnecessarily judged.
The high you get from drugs allows some people to forget how cappy of a world this is. And, if you can make it day to day just getting drugs, getting high, repeat - and all your "wants"are met - then, there is no desire to leave it.
It isn't an avoidance of seeking help; it is they know that most people won't give up two minutes to help them based on what they have done, and who they are.
Everyone does "something" to keep their mind off of the obvious - that this entire world is very close to he'll for many peoplr, and most of us are powerless to change the full dynamics despite being told we control our destinies.
Some people work out, do caffeine, smoke, drink, hike, own animals, drugs, hyperfocus, etc. Everyone has a vice for which they feel is justifiable.
Drug use just happens to be a vice that is easily visible and discernable - and therefore one of the first types of vices to be judged.