I have had a rethink on this.
It doesnt matter if Cavalry charge the square, I mean why should they. It would be suicide, besides the horses have a lot of bearing on this, as they will not run willing run into rows of bayontes.
The tactic was to force the infantry into square, of course if they could not they would route. But the main reason, was that and especially in Waterloo, there suppose to be infantry support to back up the charge! There was not. Ney charged without Infantry support!
Infantry attacking a square is far more devastating, artillery especially horse artillery is suppose to support a Cavlary charge as well, that too can deliver a far harder blow on infantry square, and then when the sqaure routs, the Cavalry just folow them up, and destroy, and or capture.
In waterloo Ney didnt enough support of either Artillery or infantry if at all!