The constraints on Japanese military operations in SE Asia, Burma, & the Pacific were mainly logistical. They never ran out of men, but they did run out of the ability to arm, supply, & reinforce them, the ability to train enough pilots, to make enough aircraft (or maintain the quality of those they were already making), to make good losses of ships, etc.
The Japanese armies in China were self-sufficient as far as possible. They largely lived off the land they'd conquered, & fought a mainly infantry war. They also recruited locals to fight for them, & from the beginning of 1943 had a puppet Chinese government to help them do it.
They had guns, a few tanks, etc., but neither most of the men nor their weapons could have got to the theatres of action in Burma, India, New Guinea, the Marianas, etc. Their aircraft & pilots might have been able to make a contribution to those other campaigns, but they'd have been difficult to deploy or keep supplied.
The Japanese armies in China were self-sufficient as far as possible. They largely lived off the land they'd conquered, & fought a mainly infantry war. They also recruited locals to fight for them, & from the beginning of 1943 had a puppet Chinese government to help them do it.
They had guns, a few tanks, etc., but neither most of the men nor their weapons could have got to the theatres of action in Burma, India, New Guinea, the Marianas, etc. Their aircraft & pilots might have been able to make a contribution to those other campaigns, but they'd have been difficult to deploy or keep supplied.