The area between thinking others are "absolutely not cheating" and "definitely cheating" is huge, and where most online gamers end up spending their time. Absolutely cheating is not a problem, because most games allow you to report people, and they'll get enough reports and be banned shortly. Not being able to tell is the problem, because you feel something was unfair (which you might also feel when it actually is fair), but you feel it more often and it leads to poor experiences and people quitting the game.

Cheaters quit after a while because the game loses its challenge, and people being cheated leave because the game.feela unfair, and eventually there's not much of a community left.