Human traffickers see orphans as the ideal target. If no one misses them, no one will come looking for them.
Get hard facts here in the 2018 Trafficking in Persons Report.
For the same reason, these children are sold into slave labor. you can learn more about that here and here.
If we really want to stop human trafficking, one powerful way is to welcome orphaned children and foster youth into consistent relationships and loving homes.
You may not be called to welcome one into your home, but you are called to pray.