However except at close range, hitting a road wheel is not really a big threat. If the enemy already has the tank in the broadside profile, why not aim for the much easier to hit crew areas? Aiming for roadwheels means aiming, aim too low and you kill fluffy bunnys not tanks.
As for tank sights, most are "proofed" vs splinters and machine gun fire in various ways. For example on the Abrams the primary FLIR is set back from the edge of the turret in an armored box. It would be very difficult for shrapnel to hit the sight after exploding close enough to keep enough energy to actually do the job. The leopard II makes the primary sight untouchable from every angle but the direct front.
So while you can target the sights, like roadwheels is a hard target to get.