Some of it is economics but some of it is the structure of relationship choice. Feminist scholar Eva Illouz in Why Love Hurts talks about the reasons why women find it hard to get into committed relationships where they feel safe having children:

https://www.amazon.com/Why-Love-Hurts-Sociological-Explanati...

Not least the idea that if you keep dating you can find somebody better than you've found so far -- a problem that's worse in large cosmopolitan cities where the dating pool is large and perceived to be large.