There are a lot more than 59 missing children at any time. There are hundreds of millions of people in the US, a few will go missing all the time.
most missing children are with their other parent who is hiding from court ordered custody, and thus don't even know they are missing. However kidnapping (often for sex) does happen once in a while, it takes time for the FBI to track these down and in the mean time those doing it have time to kidnap a 58 more kids.
there is nothing unique about the US here. all countries have similar problems, as criminals are everywhere and it takes time for law enforcement to track down any particular criminal.