Yes, I don't think this is a single faceted issue. I've yet to see anyone here mention animals yet... but numerous animals in captivity have also been shown to have far lower breeding/fertility rates. Factors like restricted space, lack of mate choice, and disrupted natural instincts can all influence animal behavior, and I see no reason why the same can't apply to us?
I know for me personally, it's not economic reasons as to why I have not had children, but more a problem of finding the right mate at the right time. Some people just aren't socially fit for each other...
I think the real interesting question isn't "why don't people have children" but more "why do those who have one or two kids not have more?" It seems everyone around came from a family of 6+ or knew those who did, yet everyone has 1 or 2 kids, three if the first two were both the same sex.
I didn't choose to have kids, but I have a friend who prioritized doing so, and she talked about hoping to have a larger family. She got married and had her first child not long after graduating from college. So biologically a very healthy age.
She ended up with two. Pregnancy sounds nice and well until your teeth start falling out. Some women just have a really rough time of it - so doing it while also being the primary caregiver for 1 or more other young child... yeah, even if you're financially stable and supported from your spouse's job, that is really a hard thing to manage.
In her case, it seems extra hard because neither her parents nor her husband's have helped with caring for the kids.
Meanwhile, my step-sister (who is less financially well-off than my friend) has 3, but they are constantly hanging out at my parents place or with extended family. Having nearby family that wants to help makes such a huge difference.
The familial support really is key - it is possible to have a large family "on your own" but it's significantly harder than if you have cooperative family in the area, or are already in multi-generational living situations.
I know for me personally, it's not economic reasons as to why I have not had children, but more a problem of finding the right mate at the right time. Some people just aren't socially fit for each other...