The reason why women are obsessed with having "bio kids" is so they can have a kid they can supply with a father figure

Beyond that is because it's a lot easier to have a kid that's biologically yours to be a single mother to because a lot of women will not date single mothers unless the kid is out the house. So they have the biological kid they can keep around and then they can date guys with no kids and if they do get kids, they can spend more time with the man's kids than the kid they had as a single mother. It would be hard for a single mom to see a single dad date her and then have to spend more time raising sons biological kids while still having her own biological kids she's raising alone.