He may have been bipolar or had an other type of mental illness. Often families just give up after years of intervention and forced hospitalizations. People like this often becomes transient because they have warrants for minor crimes all over the state or even country.
I knew a family like that. They would try to help their son and for awhile he would be in treatment and get better, but then go off his meds and disappear for a year or more. Then he would show up again.
That family did it over and over again (the Prodigal Son) to the point where it had become “normal” to them. “Yeah - the last time we saw him was Christmas a year ago and we have no idea where he is or how he is doing. Hopefully he’ll show up again and we can try to get him the help he needs.” I have no idea how it finally worked out for them.