Presumably, a lot of the 0.2% are with a parent or family member who isn't supposed to have custody. I would imagine that in many cases the kids are being treated well. Just because a parent didn't get full custody in a divorce doesn't mean they're a bad parent, just that in court they couldn't prove that they deserved 100% custody. Kidnapping a kid is a radical step, but it doesn't necessarily mean that the kid is worse off than if they'd remained with the parent who was supposed to have custody at that time.
Others are probably running away from a bad situation at home. It would be good to get them the help they need. But, it's likely that if you just spent more on looking for missing children, they'd simply be brought back home and have to live through whatever made them run away. Sometimes I'm sure the kids are better off at home, but other times they're running away from severe abuse.
There's probably a tiny fraction that are kidnapped by strangers. Say it's only 1% of those that go missing and aren't returned. That's about 12 per year in a country of 350 million. How much extra tax would you be willing to pay (and force everyone else to pay) to reduce that by 1?