Housing, childcare and education are three of the highest inflation categories outside of medical care. The more kids you have, the more of each you need, and the costs are basically linear.
Outside the 1% the numbers are lower, but it's obviously much harder when expressed as percentage of income.
2 interesting links:
https://www.usatoday.com/story/money/2020/11/20/income-it-ta...
https://worldpopulationreview.com/state-rankings/child-care-...
"The average cost of providing center-based care for an infant in the U.S. is $1,230 per month. "
So on AVERAGE, having 2 kids in daycare is $30k/year nationally. For most anyone, anywhere, $30k/year is a lot of money.
Benchmarking "average 2 kids childcare cost" in state against "top 20% income after tax" in state:
Nebraska 30%
Connecticut 26%
Pennsylvania 25%
Virginia 25%
Arizona 23%
New Mexico 20%
And again this is average childcare cost vs richest 20% (who may likely spend more than average).
People lowering down the bracket closer to average income also need childcare, which is going to consume a higher percent of income.
Sometimes this means one spouse exists the workforce involuntarily to do childcare as the cost of daycare exceeds the lower paid spouses income. Other times it means less formal forms of childcare with the associated costs/risks implied.
Some happily choose to be a full-time parent, and the money does not factor in.