I think yes, a lack of introspection. Simply 'doing' and not thinking. The fact that poly is still largely considered 'weird' - people tend to stick to what they know and many people find 'alternative' lifestyles too strange to even both contemplating.
Plus, from what I have seen and heard, people who tend to cheat, or lean towards cheating, also seem to be people that are able to live in denial. I have heard "but I'm not really cheating if [we're not married yet / it only happens once / we aren't having sex / xyz]" so many times. So, perhaps there is something in that.
My GF and I were discussing cheating last night actually. Ironically, given that we are now in a poly relationship, neither of us have ever, ever cheated on a single partner, throughout our entire life. Yet some of the people I know who find the concept of poly strange and don't give me their full support are involved in relationships with married people. Strange.
My GF has a theory that those single people who look for non-available romantic partners do so because they cannot truly cope with a meaningful relationship, on some level. Poly is ALL about meaningful relationships, even hierarchical poly and swinging, because we learn to communicate with and consider our partner. So, if they cannot cope with meaningful relationships, it's psychologically easier for them to seek the unavailable than to consider the concept of the work involved with poly.
A lot of people are selfish too. And immoral. Yet humans pass judgment very easily. So, those having affairs can KNOW it's 'wrong', and in their quest to be 'right', they keep their affairs a secret, because they plan to someday be monogamous (i.e. 'right') again. To them, monogamy rules (ironically) - so the idea of opening up their relationship just feels like openly cheating - so, if cheating is 'wrong', they don't want the world to know about it.
Me: (30f) open poly
GF: (40f) My long-term, long-distance partner
Hubby (37m): GF's husband
Garcon (26m): GF's submissive/third partner
“Peace comes from within. Do not seek it without." ~ Buddha