In the case of the SKS, it can't be helped, as the rear receiver
cover is removable and wiggles around a bit; mounting a rear sight
on it would result in an inconsistent zero.
Personally, I like the rear sight where it is on the SKS. It's far
enough forward that it remains in focus with the front sight when
acquiring a sight picture. In comparison, on an AR with iron
sights, the rear aperture is usually out of focus due to its
closeness to the shooter's eye.