I think a big part of the problem is that many men are socialized from childhood that doing anything that displeases women, that gets women upset or mad at them, is bad and therefore should not be done. Men get this from boyhood from their mothers, from (mostly female) teachers, and from their girlfriends as they become teenagers.

On the other hand, the thing about a decent exposure of an affair, is that it usually makes the wayward spouse upset, in fact it often makes them FURIOUS. Interfering in a wayward's affair in any way may bring the wayward's "wrath" upon the BS whos' only trying to save their marriage. Many men are simply not prepared to deal with a raging, raving woman threatening horrible consequences and/or saying how much they HATE the man they once professed to love. Many men can guess (or fear) this will be what they'll have to deal with, and they don't want to, because their strongly socialized don't-piss-her-off instincts tell them that this will end their marriage for sure, that this will drive her into the arms of her OM forever, never to return (and sometimes that IS the result of exposure).