Why do women always have somebody else lined up.

Why do women always have somebody else lined up before they break up with you. It seems like women mentally check out with you about a month before the breakup and then after break up within a month or two they're with their new guy. I've seen so many good hard-working men get devastated by this b*******. What exactly are women these days looking for. And I'm talking about the modern woman not the traditional mindset woman. In which those are far and few between these days