And the movement bonus, and the jousting experience, and the inability to dig in, and the inability to do anything against walls unless they have infantry siege support.
But seriously, cavalry as they are now have a nasty charge that can be devastating if done correctly. They are basically the knockout punch, so if you didn't knock the guy out then you hopefully have some contingency plan. To allow cavalry to have more than one charge might skew open field battles a bit too far towards cavalry. That might mean open fields use cavalry going against cavalry, in which case typical metagame evolution means people will try to counter with long range archers and special forces behind box formation infantry. Then that means a counter for that, possibly heavy infantry line. Then we go back to heavy cavalry charge, and so on.
It's ok for a cycle, but it just might lead to some unforeseen balance alterations. Cavalry as they are now seem to be pretty good and are clear in their purpose. If used correctly they can be the big power finisher. They could also serve as a strong opening to breach a hole for the infantry to mop up. But that is their purpose, and they serve it well.