So many hackers in games because they want to feel the stimulation of gaining reputation, currency, and resources within virtual realities. These games play with peoples reward systems by making the games challenging and providing adequate rewards to stimulate peoples reward-system and thus making them continue to play. When hacking comes into play, you basically become God in a virtual reality. Your powers are limitless and there's really no point in "playing" anymore except for playing your inevitable wins due to your god-like abilities. This is why it ruins the game for other people, because they're trying to stimulate your reward system, but you've just gone hay-wire and don't have one anymore because you "beat the game". So where does this fun come from?
People like to win and people hate to lose. In the studies of economics and decision theory, this is called Loss Aversion. Losing is more painful than winning is pleasurable. This relates back to the economic law of diminishing marginal utility. This law states that the more a person has of anything, the less pleasure they derive from additional units. These additional units can be anything including superficial things like wins in a game. So the more a person wins in a game, the less pleasure they derive from each additional win. However, if they lose a few rounds or just simply turn off the video game for a while, this resets what some economists call the person's "Subjective State" (which is just the person's psychological responsive state). Most video games are based on this principle, in that players should not win every match nor they should they lose every match. Most video games try to find an ideal balance between wins and losses so a player's subjective state stays right around neutral; thus, players derive a considerable amount pleasure from every win. Hacks, however, cause an imbalance with a person's subjective state.
When a player cheats in a game, they significantly increase their subjective state. This exploits the mental reward system and causes nearly constant dopamine release. Over time, though, a player will adjust to these new dopamine levels and then they will start to become bored with winning. In other words, they will become tolerant of the new dopamine levels and thus they will derive less pleasure from each additional win, as stated by the law of diminishing marginal utility. However, some players will continue to hack/cheat because losses are more hurtful more than wins are satisfying (Theory of Loss Aversion). If they turn off their cheats and lose a couple rounds or if they take a break from the game for a while, their subjective state will reset and then hacking and winning will be fun again.
TL;DR: People cheat because losing is painful and winning is pleasurable to an extent. There is a point when constantly winning no longer gives a player very much pleasure and at that point the player will either stop cheating or take a break from the game. After a player has taken a break or lost a few matches, cheating becomes fun again. This is all related to concepts from the studies of economics and decision theory.
I hack because it's fun to do things I normally can't within the game. Of course hacking gets boring very quickly. At least for me, it's just something I do when I'm bored. Nothing really about 'beating the game'.
On another note, why are you on a hacking forum complaining about this?