It's important to point out, on Intervention, the families are saying you cannot stay w/us any longer, we will also no longer give you money or support. So most of them are likely getting kicked out of their homes or losing their access to funds as most who end up on that show don't work.
They are also being put in a high pressure situation where a plane is waiting for them that same day. It's also done by professionals. Families who try it themselves are much more likely to cave, let them stay in the house, continue to supply money, and let them think about it for a while, which means they would likely come back and say No, I am not going to go.
I think that is the reason that the professional interventions are more successful than a family or loved one trying to do it themselves.
That is just my opinion as I watch the show often and have put some thought into it.