Well, i mean i would be all for that but in reality it might not be that easy. It could rely on dependencies that are proprietary that cannot be shipped or provided with the project.
It could alone be that the connection is hard coded in the game itself so instead of just booting up the server and being able to play you would now have to do something to the game itself too that it finds your server. Nothing really that cannot be addressed, I mean people could do that with ragnarok online private servers but not something your normal gamer could do.
Short answer: Because their motivation is to win!
I read something about this in the Book “Introduction to Game Design, Prototyping, and Development: From Concept to Playable Game With Unity and C#” by Jeremy Gibson a while ago, maybe that can explain this a bit.
Basically, every Player has some Intention or the “Player Intent” which is described by the Personality Types of Richard Bartle. For example, you have:
And then you have two others that you will be encountering:
So, the motivation to “cheat” could either be that this player doesn’t really care about the game, is able to get away with cheating and just wants to beat the game. According to Jeremy Gibson, a cheater might not cheat if they can win legitimately but I would argue that cheaters are usually not great players in the first place so the bar would be pretty low for them to “win legitimately”.
As for the spoilsport, this is extremely hard to work against or prevent because the motivation isn’t about the game anymore but other players, to make their experience miserable so that the spoilsport can gain satisfaction from it. Hence also the use of “don’t feed the trolls”.
With that being said, when you ask why someone would cheat, the question would rather be “What is their motivation” and the answer to that is “to win the game, at all costs”. And, most of the time, they will get away with this because they apparently cannot be caught as quickly as they can still continue doing it, if there is any action against them at all.