Most families can afford more children, I find, if they cut the excess luxury goods. I see wealth as an excuse not to become holy… remember what Jesus said to the young rich man…
Poorer nations have larger families…since we are rich, why don’t we?
Christi pax,
Lucretius
While it’s not the only reason, poorer nations also have accepted lower standards of living than we do.
As just one example: in, say, Niger, you aren’t going to have CPS knocking on your door if you can’t afford a car that will fit all of your kids in some variation of car seat/booster/what-have-you 'til they’re 7+ years old. SUVs and minivans, which is what you’ll need after 2 kids in 7-10 years, cost more in gas, upkeep, and so forth than a small car does.
In Niger, you can live in a one-room, dirt-floored shack with your ten kids, and CPS won’t be knocking on your door. Here, depending on which state you live in, having more than 3-4 kids in a bedroom (and after a certain point, of mixed sexes) will have CPS investigating.
In Niger, you can pull your eight-year-old out of school so that he or she can herd cattle or work in agriculture in order to bring in a bit more money for the family. Here…not so much, plus in poorer areas, you have to worry about gang violence even at the elementary school levels. In the big city nearest to me, the worst elementary schools have active gang members waiting outside and recruiting the little kids who go to those schools to work as drug couriers. When you’re hungry and scared and all of 8 years old, earning a little cash, or approval, or just protection from being beaten up are all pretty powerful motivators to get involved with gangs.
While health insurance has become slightly less of an issue now, health care costs are still at least a consideration for many families, especially those who make too much for assistance but who have less-than-stellar healthcare coverage via job benefits: if you make, say, 50K/year, and each child means an out-of-pocket cost of 5K for the birth alone, that’s a significant chunk of change. Our family has superb health insurance, but that’s rare. Newborns also need a lot of pediatric checkups, and while we pay all of $25 for a copay for checkup, the actual charge from the doctor’s office to my insurance company is $250.
This is not at all a slam on big families: they can be wonderful, and DH came from, IMNSHO, one of the best.

(He’s the oldest of 8, and neither of us would trade that for the world.) However, even just 20 years ago when he was growing up, things were very different.
I’m currently pregnant with our second. I find going to OB appointments very stressful with the older kid because I do not like having a fairly aware toddler in the room while I have a pelvic exam, or, at the risk of being blunt, when I’m bleeding heavily and having a transvaginal ultrasound to determine that I miscarried a baby*. This issue is only going to get worse as she gets older. We don’t have local family or childcare. When I bring this up to DH, who can’t take off work to watch DD, he says that his mom just left him and his siblings in the waiting room for OB appointments, or with a nurse. I can’t leave a toddler unattended in the waiting room, and the nurses/medical assistants can’t watch DD due to liability reasons, so I less-than-cheerfully forsee a time when I’ll have prepubescent (or pubescent, ugh) kids in the room with me for pelvic exams with future siblings. Lovely.