Imagine the aircraft in flight with the fuselage is effectively hanging on the wings. Now imagine applying vertical loads of 2.5G to 3.0G in turbulence.

You can either look at what happens next as the wings stay stationary and the fuselage bounces up and down or the fuselage stays stationary and the wings bounce up and down. The net result is the same, the loads on the wing root have more to do with the mass/inertia of the fuselage than the mass of the wings because the fuselage is, proportionally, so much heavier and because the lateral CP is not far off the lateral CG of each wing.

Note we’re not saying it has nothing to do with the fuel load in the wings - just less.
Much of Mass & Balance is simplified to create workable limits for pilots. MZFM is a simplified limit.