Women and men expect a LOT out of the same one person they are married to. Getting any of those needs met with someone else is pretty much OK, except for romance & sex.

Husbands who once were affectionate and attentive and met their wives romantic needs sometimes do less and less, until someone else might do it as well or even better.

Wives who treated their husbands like boyfriends and lovers begin to treat them like roommates or servants. Variety is attractive but being with someone who wants you and enjoys you is a simpler explanation.

No one would date someone who was too busy, too tired, or just not interested, but more than half the world is married to someone too busy, too tired, or just not attracted to them.

Cheating has three parts: unmet needs, opportunity, and an excuse, like no harm done, or couldn't help myself. Unmet needs are common. Men who married "every night with enthusiasm" women wake up married to "do we have to?" wives. Women whose husbands wooed to seduce them, wake up married to "take them for granted" husbands. The biggest difference is opportunity. Young attractive wives who work around men face temptations. Successful men with power, time & money will have more opportnity with age.

One question is why cheat instead of break up? Is it hope the inattentive spouse will get better? Is it fear the on the side lover will turn as inattentive as a spouse? Is it just to share parenting or avoid splitting the money?

"Is it just to share parenting or avoid splitting the money?"

Yes and Yes. Just? Those issues seem to be - probably - what hold ninety-percent of marriages together until after middle-age, by my very limited ability to estimate such things.

Plus some people are financially tied to their family by some way or another so when they quit a marriage, it's not just about two people's finances. Also, it's awkward socially, to have a divorce I suppose. I would think. But maybe not. Maybe only to religious people or bigots.

I agree, in most cases one cheats rather than leave out of a fear of that complicated financial untangling - also of course the emotional damage to spouse and to kids, and the fear of being pummelled by spouse or their collective social ring. I cheated - not within marriage but within a relationship in which monogamy was expected, if not contractually agreed to (seriously, how many times have you and your partner actually felt the need to spell it out "No! No you may NOT have sex with anyone but meeee"?). Anyway, when I did cheat, it was with no intent or desire to leave my partner. It was simply out of the curiosity and novelty that drives most of us to scratch the itch. I must say that scratching it (in my case anyway) actually gave me a more solid feeling about my boyfriend, and a fear that I would be found out and lose him. Eventually I did - but to another woman HE had been seeing. 15 years later btw, they are still together with three kids. And happy? Well, who could know?

I would venture that, in many way, marriage is a practical institution, held together by practical considerations. There is a good deal to be said for negotiating a mutually beneficial long term contract with another individual for the facillitation of mutual prosperity and stability. That being said, it is possible that marriage comes with a set of unreasonable expectations as well, expectations that humanity cannot be expected to meet based on evidence. Would it not be wiser to modify our expectations to avoid all the disappointment rather than continuing to hold ourselves to a set of criteria that historically we have demonstrated ourselves as unable to meet?