Prevalence of dual income families. This not only increases the demand for things like childcare by sheer number of families requiring it, but also by making it so that those families are able to pay more by having a higher income.

Shortage of housing, at least in western countries. For whatever reason, housing production isn't keeping pace with demand, probably due to regulations and NIMBYism.

Shortage of quality educational resources. Unsurprisingly, in such a heavily regulated sector, supply isn't keeping up properly with demand because of the lack of price signal. Some of that price signal is actually loaded onto the housing market, because of attendance zoning. But because the revenue of the housing market isn't always tied directly into the education suppliers, there are all sorts of incentive misalignments.

Braindrain of female workforce from educational sector. I heard this thesis from someone and don't know how true it is statistically, but it seems to make sense. As more jobs have opened up to females in different industries, one side effect has been a drain of female talent away from teaching. IMO the solution isn't to go back to relegating females to teaching, but it's to increase the desirability of the teaching profession overall.

Outdated expectations. All of the above could still be affordable for most people, if they're willing to spend a huge fraction of their income on their children. In fact, most of us could probably do that and still live a materially more abundant life than our parents (except in terms of housing, probably). But because most of us grew up in families where it wasn't the norm to spend so much income on the children, we see that as abnormal. I think if people reorient their financial expectations, it would feel less daunting to have kids.