Why forced labor could not solve Japan's fortification problem

Why did Japan not use its large pool of forced labor to fortify more islands?

The labor was not where it was needed, and Japan lacked the logistics to move and sustain it across the required island positions.

The constraint was not Japanese reluctance to use coercion but the location and movement of labor. Workers were not necessarily near the islands that required defenses. Moving them, their guards, construction materials, food, and equipment would burden the same strained maritime network the fortifications were meant to protect. A larger program was therefore logistically difficult and would also have intensified the suffering imposed on forced laborers.