It isn't that people don't care exactly.
There are numerous, thousands, of exceptions. But many prostitutes and their male counterparts - homeless and petty criminals - will have grown up in foster care so family relations severed right there. They will have run away numerous times before turning 18, and only half hearted attempts will be made to find them. They will stop attending school or be expelled. They will be introduced to drugs, may be evicted numerous times from accommodation, be in and out of prison.
It's hard with the best will in the world to keep tabs on people like that, and that line is 'said' with compassion.
I was at university in Manchester and once offered a prostitute a lift. Daft retrospectively but i was 18 and pretty daft. She turned out to be the saddest, most broken person I ever met. Really sad.
I know a few times when younger I did low paid shit jobs and a few times I just didn't go back as I couldn't face it. Maybe they would ring my phone a handful of times but that was it.
All it takes to vanish is to only sub exist in the first place.