Why do people cheat in games esp the online side of them ?

Ok some people only cheat to get past a certain part they cant manage, or to get a newer car so they can win more points . It spoils the game for the others when you cheat online so why do it ? It also spoils the game for yourself if you cheat on and offline . I was just playing Mario Kart Wii online and the others i was playing against were clearly cheating . Finishing the lap in 20 seconds, everytime i overtook someone my race position never changed ( even tho i had not been lapped )