I did once believe in that logic when I was a kid. When I grew up gradually, I waited frigging 75 years to see if my grandfather actually come back to earth and meet me and my siblings again BUT no I don't see any body of gramp in sight. So much for Hmong belief.
How do you know he didn't get reborn into one of your families as a baby? By argument of faith, no one can prove otherwise. If you had enough faith to believe that he did then he did, if you believe that he didn't then he didn't. That is what faith is, you either belief it or you don't. You can't really know whether what you had faith in gets to be real or not, sometimes what you had faith in or not is both wrong and or right. No one really knows about this faith stuffs.
OR maybe your grandpa chose not to come back and get reborn or chose not to get reborn into the same family. Maybe he wants to try other families?
Maybe your grandpa chose to protect your families from the great beyond?
Maybe your grandpa is waiting to meet you in heaven?
Faith is weird when you base things on faith alone but that is why it is call faith and belief.