[tsycho]

It's not just about money. Even if parents can afford to have one of them be stay at home, neither parent might want to be the one staying at home.

Being a stay at higher parent is an incredibly difficult, stressful and under appreciated job so to many well educated couples, it's not appealing at all.

[Kluny]

True that - not to mention tedious, mentally understimulating, and comes with a perceived lack of social status. Even for people who love it and can afford it, there are many downsides.

[jazzyk]

How about having one of the parents work part-time?

True, not all jobs can be offered as part-time, and working part-time limits your career growth if you work for a big corporation, but still, it is a viable option.