It is a bit of all, never met a cheater in person but have seen some in forums and FB groups, they cheat because
A) They are too lazy to walk
B) They think they are smarter and better than the rest, so their success is to get everything with less effort than the legal players and then show it to the world, like small kids when they say look dad i can go to the bathroom alone...
C) Some of them think that because they can't go out to play or their town has not a lot of stops, gyms and spawns they are in their "right" to cheat to get the same as big city players (of course end up getting a lot more than anyone)
D) They have no other thing to do, they have no smartphone or their phone can't play the game so they think I will play from my couch and laugh at the players that are on the street walking, the game was pretty famous at launch and people with a lot of spare time probably gave it a try and then start cheating when they got bored because they have to actually move and walk to get things done.
E) Because they can cheat, Niantic let's them do it so why not take advantage of it? "I have seen those exactly words on a FB group when a cheater was asked for it, he had more than 70 gyms all over the city and the nearby towns..."
Those are the main reasons that come to my mind right now, but there are many others and similiar ones.