Again, why shouldn't they? 2.5 children is a perfectly acceptable number.
Also, the birthrate in the USA (and most Western world) is already below the replacement rate for decades now. [1]
[1] https://www.iheart.com/content/2019-05-15-birth-rate-in-unit...
EDIT: Sorry, I wasn't understanding your point that families can't afford to have as many kids as they could one generation ago. I agree with you.