Did they really have such trouble with supplies that no significant number could have been moved to other theaters?
As xchen08 as explained, supply issues meant Japanese troops either literally starved or were on starvation level rations across the Pacific. The food deficit was so critical that, while no where near commonplace, cannibalism among Japanese garrisons wasn't exactly rare either.
Taiwan was an exception, but it was a pre-war colony. The DEI was a partial exception as was the Philippines, although Japanese troops in certain parts of those island chains did starve too. The food situation in Burma varied depending on Japanese plans. Not enough food could be seized locally so, when munition stocks needed to be built up for attacks, food rations were cut due to a lack of transport.
Elsewhere across the Pacific, IJA and SNLF detachments either faced a quick death from Allied attacks or a slow one from a lack of food. Even when the IJN controlled or just contested the waters around a garrisoned island, such as in the beginning of the Solomons campaign or during MacArthur's advance along the northern coast of New Guinea, Japan could not get enough food to the troops she already had in place.
Cramming more troops into Pacific islands will simply cause an already broken Japanese supply system to fail much more quickly.