The reasons are many, and vary greatly from person: sometimes gameplay is unsatisfying, bland plots, wrong game length... list goes on.
I usually play games only for their story, so i never hesitate in cheating as soon as gameplay feels like an obstacle.
But the only cheats that i usually use is about infinite HP/Gold/Skill Point... whichever feels necessary to progress and only when it feels necessary.
I'm not fan of easy patches because i like to feel the frustration that i was supposed to feel (I count it as a part of story), but just fast-forward it with help of cheats.
For these reasons, anything gameplay wise, won't make me quit the game, not unless its so tedious that even cheating or using walkthrough can't help it... (there was a few!)
My biggest issue is I have downloaded/purchased WAY too many games at one time...
My backlog is just too full too...
I can completely relate. Forcibly focusing down on at most two games at a time helps (to avoid burnout), preferably different styles.
I always only play one game at a time, no matter how much i like other games in my backlog, or how tedious current game is.
I won't stop until at least finishing its main story... but depending on the game, i might or might not play the post-ending contents, if any.
And i usually won't play big games after each other, i usually play a big serious game to end, and then some smaller light-hearted ones, and then after enough rest, next big one.
I think this method helped me to both enjoy games and maintain brain power for other works.