Biomechanical differences between screw-retained and cemented protheses?

I wonder if you could clarify something about screw and cement retained implant crowns and bridges. I have heard that screw-retained crowns and bridges move the fulcrum to the bone level so they transmit more stress to the bone than cemented crowns and bridges. The cement retained ones dissipate more of the force, so they cause less crestal bone loss than screw retained. If this is true, how come we do not see cement retained All-on-4 cases?