Why women think their husbands should care about the children she has from previous relationships as much as he cares for his own biological child instead of accepting that thanks to bad decisions they're 100% for a child?

Because it is, simply put, not the child's fault. All children need love. I did not have children before my husband and I married, but he has one from his first marriage. I love his daughter. Not caring for children no matter where they come from, IMO, makes a person heartless, and not anyone I would want to associate with.