It’s no different than Matt Lauer (remember him?) or Harvey Weinstein or any number of rich and famous. I think they think they’ll get away with it, nobody will find out. It will be our little secret.

They are used to so much power over others, receiving no repercussions for their actions, that they inevitably convince themselves that they are untouchable and above the law.
And unfortunately, that is often true.

Many of them got to be where they are by being compete psychopaths. Statistically, a lot of them are.

Oh and don’t forget about Elon and his spreading of lies.
They think they are above everyone and everything.

Your question seems to include an implicit assumption that we live in a meritocracy where the best people rise to the top. I disagree with that assumption. Plenty of amazing people never get that lucky brake and many incompetent people “fail up.” In some industries instead of selecting for competence, often the traits that get rewarded are dishonesty and willingness to ignore ethical/moral/legal concerns.

For the same reason poor people do stupid stuff like that. The 7 deadly sins. In the case you cited it isn’t clear who he had a relationship, if he was already married, if it had anything to do with anything other than an ethics violation (in other words was it totally consensual or did he force himself onto some poor girl), or if they are just looking for anything to use against him to get him out of office. But assuming he is married and was having an affair, or even that he was single and was having an affair with an underling, you can pretty much throw Lust and Pride as the two things that played into it.

The typical CEO who avoids sexual misadventures… still tends to be pretty stupid.
CEOs of public companies tend to have charisma and some management skills. And that’s about it. They are not very sharp people. (I’m heavily generalizing. Some CEOs are geniuses, especially founder-CEOs such as Martine Rothblatt).