Despite what some people may think, motorcycle roadracing is not only a highly strenuous sport, but there is MUCH more involved than just twisting the throttle and turning the handlebars.
As you can see from the picture, there are a number of powerful forces working on the bike and the rider. There is, of course, the downward pull of gravity, friction between the tires and the track, and centrifugal force which acts to the outside of the turn. The key to cornering at high speeds is to perfect the lean angle of the motorcycle so that the force of gravity reaches equilibrium with the centrifugal force attempting to stand the bike back up. If the bike is leaned to far, it falls over. If it is not leaned far enough, centrifugal force pulls it back up and the turn is not sharp enough. If there is no lean at all, extreme enough circumstances could even cause the motorcycle to tip over in the opposite direction of the way it is turning, much as a four-wheeled vehicle will tip if turned too sharply and quickly.