I see what you're saying, but there's something you're overlooking in your logic. You're looking at an airplane sitting on the ground, where the wheels are near the fuselage and most of the wings are dead weight that creates strain on the structure.
Think about one in flight. Now all the lift is coming from the wings, imagine the airplane suspended by a couple dozen (billion) cables spread around the wing surfaces. Now the fuselage is dead weight and the strain in the structure is from carrying the fuselage.
So when you add weight to the wings evenly, it adds practically zero structural load for the wings. What's being lifted is inside the source of the lift. So from a structural load perspective, it's a wash: it doesn't matter.
Whereas if you add more tanks in the fuselage, that's fine on the ground, but it adds huge stresses to the wings in flight, effectively reducing practical cargo capacity.
The strain on wings from sitting on the ground is much less worrisome to designers than the strains in flight.
See also "Zero Fuel Weight".