1. You're alpha and chicks would rather have part of an alpha than all of a beta (see also: Tiger Woods).
2. Prince Charming Theory. You have options and you chose her as the best of the best, so she's supremely validated by you picking her.
3. "Living well is the best revenge". Her knowing somewhere in the back of her mind that if she ever did leave you that she'd be easily replaced, and probably by a younger, hornier, better looking woman that it negates the smug satisfaction "he's a loser, he's nothing without me" blah blah blah that women would use to rationalize feeling better about a breakup. The realization it's no sweat off your ass and in fact she could be doing the man a huge favor, well, that's the nuclear bomb of psychological warfare on a woman's mind.
1 makes you in demand. 2 gives her validation and security. 3 gives her fear of loss and removes fear of loss from yourself (at least in her mind), so she'll be drawn to you because in her mind you have abundance and that goes back to the first point about being in demand. It also serves to keep her in check where most men fail.