People make their own choices, and it’s not up to me, nor you, to make assumptions on their lives. If children give you fulfilment, god speed to you. If others can find happiness without children, god speed to them.
By the way, I’m speaking as a person who wants children. But I totally get my child-free friends. I know people in their 60s as well, who debated this question and found a life for themselves. There is always a “what if question” hanging around, but all in all, they’ve weighed their options and are generally happy.
I think a lot of people who ended up having children to find fulfilment did not find happiness in other means. So they can’t experience the “other side’s argument”. Same applies to child-free people, as they haven’t experienced the other side.