In general I find it hard to believe that people would choose to become parents for the sake of a potential child, who will only exist because they decide so

I'm sure many of the siblings of our ancestors chose otherwise. But I'd be surprised if that choice stayed popular.

unless they expect to enjoy raising a child at least some of the time, or otherwise profit from it (social approval, commitment between married partners, support in old age, government aid).

Those reasons help. But our instincts give an extra boost when it comes to both expecting and remembering.