If this is the link you're talking about, your numbers are way off - https://www150.statcan.gc.ca/n1/pub/11f0019m/11f0019m2023007-eng.htm
That shows that even for low income families (<83k/yr), they spend an average of 14,000$/yr on each child. That's way higher than your estimated 30,000/ages 0-7, so I'm curious where you got your data from.
I don't discount that there's a societal push for people to get older and make sure they're confident in wanting kids before they have them, and with low cost birth control we've reduced accidental pregnancies, but cost is still an enormous factor.
Developers costs go up if they try to make houses faster- increased labour costs by increased demands, plus decreased housing costs when they go to sell.
Why would they make homes faster?