Sure there are offset scopes, but they are hardly used, and not as reliable.
Huh. Funny you should say that, an offset scope mounted to the side of the receiver worked out just fine for the Mosin Nagant + PU and the Karabiner 98K which both load from the top. And somebody already linked us to a
video of a Garand with glass.
Slightly to one side, it's worth noting that all relatively modern Soviet and Russian small arms have scope mounts on the side of the receiver, not the top (such as, for random example, http://www.canonshooter.com/photos2/ak103k-5.jpg). These probably don't count toward defeating your fallacy though, since they have detachable magazines that insert from the bottom.