Because women have a limited window to have children and have to work with the options available. A woman can’t put off kids indefinitely waiting for partners to stabilize financially, emotionally etc.
that window is at least 20 years wide. if it's not possible to find a reliable partner in that time then the world is in a very sorry state.
i accept that it can be difficult, and that people can be blinded by love but if i already know that a person is unreliable enough that i don't want to marry them, then why have a child from them?