Because if cheating is done right, it is hard to proove. Most competitions allow a degree of "Failure" in the game. So if you are running a race, while you are not suppose to come in contact with the person you are racing with. However most judges will not be hardass enough to discredit an accidental hit. However if you are going to cheat, you may "Accidentally" hit your biggest threat just to get him off his stride.
Now this is cheating, however it is hard to prove.
That and sportsmanship lesson one, was to publically not be a sore loser. So saying he cheated is often the call of a sore loser.