I think I figured this out. And it's not because "men care more / are better people" or that "women are trash" etc...
I think in general, most women have never been with a man that they really really like. Note: I said like, not love.
Most men simply aren't that desirable. Most of us are unattractive, boring, don't have a lot of resources, or some combination of the 3. Most of us are not going to be a woman's first choice. She will, in her mind, be setting on some level for about 80% of men, while still fawning over a 20%'er from her past.
Now, if you're a very desirable man, one of that elusive 20% of men that all the women desire, I can guarantee you she's not quickly / easily moving on from that if you dump her.
But... most of us aren't that. So when a woman gets dumped by an 80%'er, it's easy for her to convince herself that she can do better. And that feeling of being able to upgrade goes a long way towards mitigating the feeling of loss.