While we like to think that all families are loving and caring, and everybody has friends from high school etc., the sad truth is that there are hundreds of thousands of young people for whom that's not true. They age out of foster care and have nowhere to go (my local paper had an article last week that said up to 75% of foster children turning 18 had nowhere safe to live after they were turned out), they live in abusive, violent, or drug-riddled households where they leave as soon as they're old enough and if anybody notices, they don't care, sometimes they're even pimped and sold by their own family members, and sometimes they're thrown out for lifestyle issues like religion, sexual orientation, politics.
Or sometimes the family is just too big or too poor, or there's illness or mental/emotional problems that make it impossible to keep track of everybody. I had a friend in high school in that situation -- her mother loved her dearly but she had eight siblings, her father was dying of cancer, her mother worked two jobs besides the kids, and when my friend met a guy, she was glad to get married and leave and her mother was glad to see her "settled" even though she was very young. The marriage ended after only a couple of years, my friend got a job, went to night school, eventually got a teaching certificate and a better job. She used to occasionally talk about sending her mother a Christmas card but I don't think she ever did. I imagine her mother thinks of her, too, now and again.