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:

• The Achiever who seeks to get the highest score in the game and wants to dominate it
• The Explorer who seeks to find all the hidden places in the game and wants to understand the game
• The Socializer wants to play the game with friends and wants to understand other players
• The Killer who wants to provoke other players and wants to dominate them

And then you have two others that you will be encountering:

• The Cheater who only cares about winning and does not care about the integrity of the Game and they will bend or break the rules to win
• The Spoilsport who doesn't care about winning or about the game but rather will break the game to ruin the other player's experience

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.