If you have more children, you can’t give the same amount of individual love and care to each. That said they can give love and care to each others, though not with the same level of experience and self-control as (some) adults can do.
Yes love and care are tremendously important to foster mentally healthy people.
But this alone won’t teach them literacy, swimming, mastering of self attention, math, music, biology, active listening, critical thinking, humility, sane cooperation at scale, and so on. For all that you need a safe generous educative environment that could be logistically scaled to all humans but that we will fail to put in place as we let the most capricious wealth-drainers deteriorate harmony in societies.
My attempt to explain would be a high income family aim to provide their kids the same opportunity, and they know with seven children they won’t be able to provide the personal attention to provide that. For example, consider how parental involvement in a child’s schooling is the number one predictor of educational outcome. Well, you can’t spend thirty minutes each night helping each of seven kids with their homework.
They are, but in a different way. They worry if they can still afford to pay the mortgage while also paying for a second daycare/preschool tuition and then later a second private school tuition.
> The cost of raising a kid might go up with income, but probably not at the same slope as income, which would mean they get cheaper in relative terms.
FAFSA allows colleges to perform perfect price discrimination, so the cost of raising a kid through college absolutely goes up with the same slope as income. When colleges know you have significant retirement savings and a high income, your kids will get zero financial aid.
You seem to be trying to model social phenomena as linear functions. That's going to fail on almost every time.
The GP is also claiming monotonic behavior. Please understand it as constrained into a "reasonable" interval that he didn't disclaim. Otherwise it certainly won't hold either.