It isn't easy for cavalry to charge, circle around and charge again. After a charge, horses need a wide area to turn around and run away so they can charge again. Even if they knock over humans, there's still the matter of them tripping over. Try running over something half your height and a tenth your weight. You'll knock it over but you'll probably trip, too. Physically, the difficulty of knocking down humans rises exponentially the more there are behind them, especially if they have a low center of gravity and are braced for impact. Hitting them would mean the horse would stumble.
Frontal charges just don't work against experienced, high-morale troops in a tight, deep formation, pike or no pike. Obviously, the Mongols had lots of success but horse archer combat is vasltly different from charging knights.
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