Why are center tanks not used?

Mxsmanic wrote
Why does it seem that wing tanks are filled in commercial airliners in
preference to center tanks? What is the advantage to filling the wing
tanks while keeping the center tank empty or partially filled?
Wouldn't heavy wing tanks increase the inertia of the aircraft around
the roll axis?

In order to reduce the bending moment on the wing spar at the
juncture with the fuselage. Longer airframe life.

Bob Moore