If you look at this thread (love some of the answers by the way) you will see that overwhelmingly the APs do not begin to compare, and that's before you even consider they are a POS AP.
In my case the OM was, factually, less successful, less well-off, was in the midst of a messy divorce because his wife couldn't stand his self-centred behaviour any more, was depressive, owned an apartment in massive negative equity, had few friends, no social life and had become embittered with life.
I score higher on every scale. So clearly that's not the issue.
They affair down because they need a person who provides all the ego stroking, adulation and validation they crave, and who puts up with a second-hand, cheating version of themselves.
And you get that from your inferiors, not your equals.
As a final point, it's also why once a WS comes out the other side they commonly and genuinely say "I can't believe I risked everything for..."
I'm three years out now and while I still would not pee on the OM if he was on fire by the same token I rarely think of him and I certainly don't compare us in my head - he/she are not the point, our WS are where the attention needs to focus.