I get the IC rationale for it - that one lance can't pierce five guys at the same time. But cavalry are expensive to recruit and maintain and the main benefit from that is they do extra hits on a charge. If you lessen that by wasting half those hits, then what's the point of recruiting cavalry?
Cav units tend to be smaller...That means they are going to tend to only damage one or two units at a time, unless their enemy has a lot of small units.
That makes sense, but doesn't actually seem to happen in battle. The only cavalry unit that hit more than one enemy unit in the initial charge was by far the smallest one. And there were a number of examples of smaller infantry units (15-30 men) hitting more than one enemy unit.
