Because in order for that to make sense the player character would have to be Chad.
The problem is most players cannot identify with Chad as they are the opposite of Chad.
They are the Betas that are getting cucked not doing the cucking and any reminder would trigger their IRL insecurities.
In fact how most Harem Games work is there is no male competition in the game at all, the human male population has gone extinct and you are the only dick left in the world.
Your making another one of those look down on others comments that isn't even true.
The most popular characters to self insert as are the Chads, the Geralts of Rivia, the Ezio, the Batman, some of the most well written and highly regarded games, Being a Dik and Superhuman have Chad MC's with friends who fuck.
The reason its common place to have the MC with no strong personality and almost no other males, is because it's easier for inexperienced writers to pull off, let the focus be the females.
Are you saying Harem players are insecure and this is why they remove all competition in their games and choice of games? Because they inwardly fear they couldn't compete in the real world and thus don't want it in their games?
This again falls back into inexperience of writers, even romance manga, which is infamous for how Asian audience values purity, where it even extends to their pop stars with some needing to have this persona of being pure, almost always has competition. But you can't just throw the competition in, you need to develop the story and world that most devs either don't have the time or skill for.
Some devs do try, but there are a number of games that are hampered by their attempt to create a broader story, of course I don't think they should stop trying, but if you want to work on it create a story first for a fanfiction website, a story without smut, developing the characters and if it stands on its own, you can then combine them and make something that's greater than the sum of its parts, a well written story with smut.