The bay area M/F ratio is extremely skewed and the dating market is extremely competitive - (has way too many men).

I would guess that in cities with high skew (in either direction) this creates a disincentive to staying together for one person in the relationship.

I'd also guess the NIMBYs and extreme housing cost also incentivize delaying things.

If there was a lot of housing supply the financial risk of having kids would be way lower.