When you're playing a single-player game, you still play against another player... the computer.
It throws challenges at you, and you overcome them, or you lose.
The computer imposes rules, and it's up to you to win
without breaking them.
You
could cheat, but then you would be breaking the rules.
If you were playing against a human player, they would be offended, call you out on your cheating.
Normally, the computer can enforce it's rules on you, but sometimes things can happen that aren't accounted for.
These glitches, bugs, and oversights by the developers can sometimes by exploited by players.
If you take advantage of an "exploit", the computer can't account for this, and thus
can't catch you cheating.
Against humans, you would be disqualified for cheating, but the computer will let you keep playing.
If you didn't cheat, you would have accomplished something you could be proud of yourself for, even if only a little.
However, since you've cheated, if you still go on to win, it wouldn't mean anything... your victory would be hollow, at best.
If you keep cheating, you will
always win, again and again, and again, and
again...
Since the game could no longer challenge you, there wouldn't be a point to playing, and you would inevitably grow bored and stop.
You would then put the game back in it's box, stick it in your closet, and never look back.
You would buy a new game, and you would play it, and have fun... until you found a way to cheat.
You would begin to cheat more and more, until this game began to bore you... and then you would toss it aside...
It's a vicious cycle, one I've seen before in other gamers, and that I take care to avoid myself.
I love Morrowind, it's my favorite game of all. (Hence why I'm willing to spend so much time and effort modding for it.)
As such, I avoid using exploits... I don't want to ruin the fun.
On the other hand, I want to be able to just relax, and play the game, without having to worry about such things, you know?
Thus, I would prefer that the game handle as much of the rules enforcement as possible.
I'm here to play a game, after all... not waste time worrying about my "self-discipline".