Why does it seem so important to some people that children resemble each other or a parent?

Rick and I were watching a program about MJ’s kids, and he said “Man! They look just like him!”
I looked at him in astonishment. I said, “Rick! They aren’t his biological children! Look at them! They don’t have a drop of black blood in them! They don’t look anything LIKE him. If there is any resemblance, it’s because of all the facial reconstruction that he underwent to look more white, not because of any genetics,”
Brief argument ensued. It was strange. It never, ever crossed my mind that they were his biological children. It was so obvious, to me, anyway.

My daughter had fraternal twins. A boy and a girl. You can hardly find two babies that look so dissimilar. Kale is darker skinned and has dark, almost black eyes. Savannah is white white white with startlingly blue, blue eyes. She’s a little bigger than Kale too. I posted a pic of them on fb, and tongue in cheek I said, “They don’t match!”
At which point I got a lecture about the difference between fraternal twins and identical twins rolls eyes and another friend proceeded to tell me how much they DID look alike. I got the sense that she was upset at me for suggesting that they didn’t look alike!

The only resemblance they really have is that they’re exactly the same age. In that regard, ALL 6 month old babies resemble each other.

Is it some old cultural thing that causes us to want to reassure each other that the kids have the same father, or that the child actually IS the biological father of the child? Or what is the deal with that?