It counts one parent taking a child without the permission of the other, usually during nasty divorces and custody battles. Those get resolved easily but the kid is still counted as missing.

It's because a missing child is not the same as an abducted child.
If you don't know where your child is, you tell the police. They radio the miscellaneous patrol cars and they inevitably find the six year old trying to walk somewhere silly within a few hours.
That's a missing person report.
Divorce going poorly. One parent thinks the other shouldn't get any custody. So they pick the kid up from school when it's not their turn and go to their parents house. That's a missing person and child abduction.

Basically, it's a category error. We're All tracking the same things, but our huge category has the same name as their narrow one.

Apparently 90% of missing children are runaways. Maybe that is more common in the US or maybe it's just differences in counting (for example some countries might count 16 or 17 year olds as adults for these purposes).

I think that’s because Americans are more likely to call the cops in this situation

And the ones who stay missing are often ones who were being abused by the parent and/or had a LGBT identity their parent refused to accept. They weren't kidnapped. They fled.

50-60 not found alive or not found about 1 in 10,000. Vastly less than the number who commit suicide OD or die by guns

and that's an acceptable number