The lances of the late medieval knights were also attached to armour and saddle giving them the same advantage to concentrate the kinetic energy of the horse knight combination on the tip of their lance. Plus medieval knights rode bigger and heavier horses than any ancient cavalry. But personally I don't think that this is a great advantage on the battlefield. So they can only
attack tragets directly in front of them making the later knights much less flexible in combat, and searously limiting their possible tactics. As you said flexibility, manouverebility and discipline is far more important for a powerful battlefield cavalry. Hetaroi even didn't use saddles at all, limiting the maximum power of the shock without decreasing their overall effiency. Without doing this Roman Contarii or similiar equiped cav. could fight enemies with their two handed lances on both sides and even beneath their horses and their blow is much more difficult too predict for the enemy than from the straight charging knight. Shorter 1.8-2.8m lances could alternativly be used overhand to stab behind the infantries defence and enables to aim at weaker points more easily.