Because their not in love in the first place, i mean could/would you really cheat on someone you really loved?.
I know plenty of peeps that thought they was in love, only it was just their ego minds in love with the idea of being in love.
Love is felt through our hearts, and not thought with our minds.
So the vast majority of peeps i know don't wait to find love, and they let their ego mind force them to find someone quick.
No one wants to go to a party on their own etc, and it is nice to be in a relationship with someone, also it's very handy!.
So true love like i say is a very powerful feeling, and true love will always conquer the ego mind.
So if someone cheats they was either not in love, or they've let their ego mind gain control over them, by falling out of love with their partner.
Either way i still say you can't cheat on someone who you really love, but that's only from my own perspective obviously. :-)