Why? Because one of the main historical purposes of flanking units was to hit the enemy archers/artillery.
Flanking cavalry maybe, but there was an equally valid and deadly purpose in collapsing the flank of the enemy's battle line. Infantrymen get very skittish when they are attacked from the sides, or even worse, from behind. Units being flanked often broke and ran, setting off a chain reaction down the entire line. Then the cavalry rode them down while they fled.
