Western countries with their lack of intergenerational support and lack of affordable child care while we have stagnating wages are probably the major reason.
That's your ROI; you spend time and energy raising them, caring for them, cleaning for them, providing for them. In return, they give you affection, love, you enjoy playing with them, or having conversation with them.
If you are someone who does not enjoy spending time with children, or do not think you will enjoy seeing them grow, you usually... avoid having children since you do not see the return.
At some point, you made the decision to have children because you think you would be happier with some.
The first one was actually an accident. As is so often the case. Life cannot always be explained using an Homo oeconomicus.
However, before you are in that situation, ROI calculations absolutely do apply. To give you an example - most people on HN that have children probably have 1-3 of them - and that's about as many as they wanted to have. But if you somehow happened to already have 10 children, would you then be selling 7-9 of them (or indeed any) to the circus? Of course not. You would love them, care for them and do the best you can, and you would miss each of them dearly if they were gone.
And yet, given any degree of control over the situation, you would probably try not to have so many. Why not? Because of some calculation - let's call it the ROI calculation. It is precisely the same calculation that keeps you from having 10 children that keeps others from having any children at all.
Did you enjoy spending time with other toddlers so much, that you thought you would enjoy it more than all your other hobbies (which you may have to give up), that you wanted to be doing it during the majority of your downtime, have sleepless nights etc.?