Men primarily cheat out of boredom.
Once you've been with the same woman for 3, 5, 10, 20 years, whatever, most men crave new p*ssy. The main reason all the sugar daddy sites are all the rave right now. Probably some simps, but it's mostly married guys with $$$ looking to bang young hot women because wifey is just too boring, got fat, or turned into a major b*tch. I've known women who were on these sites and that's what they told me.
My last gf of 9 years was a hot Croatian woman. About the 3rd year in, I started to get sick of screwing her. By year 6 it was a chore and not much after that I was definitely interested in pursuing sex with other women. I liked being with her and we got along, so I stayed in for a while more. I was hardly "desperate," just wanted some variety. Eventually, we broke up because we got to be more like roommates. Probably mostly my fault because the attraction just wasn't there anymore for me. Plus, she had a HUGE superiority complex which could be a real buzz (and boner) kill.
I love hot fudge sundaes but if you have one 3x day, at some point, you'll get sick of them.
Other men cheat because their wife, who may still be hot, turned into a f*cking b*tch. I've seen this many times. Or they develop personality traits that kill any attraction; narcissism, nagging, laziness, whatever. I've dated super hot women but after a while their personality was a huge boner-killer, so I dumped them.