Why can’t level D simulators produce a G-force effect?
They can produce sustained G-forces, by tilting the sim without the occupants noticing. Of course, this tilting of the 1G gravity vector is limited to practical angles of about 45°, producing 0.7G of simulated sustained horizontal acceleration. 

Vertical sustained acceleration is a different matter altogether: the 1G is always already there and can only be reduced by tilting the sim, while in manoeuvres with increased load factor we're looking to increase beyond 1G.

Fortunately the inner ear sensors can be tricked to an extent: they are very good at detecting acceleration onsets but get flooded by sustained acceleration inputs. Our brain seems to rely on the inner ear for detecting the first onset of acceleration, then on our peripheral vision to read the motion velocity. So if we heave the sim rapidly and show the movement on the visual, we cannot detect very readily if we're not pressed down by g-forces anymore after some time. 

Image source

Level D simulators are for civil aviation training, which teaches how to limit load factor. Military sims are a different kettle of fish, as mentioned in this answer. G-seats like depicted above provide cues to our body skin sensors, as do G-suits.


                
            

Centrifuges can of course produce sustained g-forces, and they do exist, used by the military to train sustained g flying and by medical facilities to check how long a person can remain conscious. A combination of a centrifuge with a civil 6-DoF system is not really possible for realistic acceleration simulation.