Men are generally raised to be good and loyal. They are taught that they shouldn't cheat on their wives, shouldn't leave their wives for younger women etc etc.

Problem with this, is that men who are loyal to a woman don't have motivation to improve themselves in terms of dating. They let themselves to, get fat, stop going out and flirting with other women, etc. Which makes them less attractive, which leads to marital problems.

Now you can say that men should keep in shape and top of their game regardless. That's fine but men dont have incentive to do so if their reward is an aging wife. There is no point of being a billionaire in a deserted island.

Wife also doesn't have enough motivation to work on herself if she knows that her husband is not going anywhere.

If men are allowed to cheat and replace their wives with better women, then it's better for everyone.

Men will put in the work to remain attractive and wives would put in the work so that husband doesn't jump ship.

The expectations of loyalty and commitment makes people careless and neglectful partners. This leads to lots of resentment because on one hand you are not allowed to leave and on the other hand, it disincentives the partner to put in effort which leads to dissolution of marriage anyways.