They never made it into central China. Chungking (Chongqing) is Central China and it was the wartime capital.
The Japanese nomially controlled about 1/4 to 1/3 of the east coast of China, and even then they only really were firmly in control of the main cities.
The Japanese stragedy seemed to rely on the Allies exhausting themselves. The main idea was to grab as much as possible while the Allies were busy in Europe and then negotiate a peace with the West and trade some of what they grabbed for peace.
The Japanese really only came into conflict with the USA because of oil. The Dutch East Indies (DEI), now Indonesia were rich in oil and Japan had to have it. If you look at a map and trace the route from Japan to the DEI you see Manilla and the Phillipines are sitting right like a dagger. If you look a bit closer you see Singapore, the “Gibralter of Asia” is also sitting right next to the DEI.
Japan simply could hope it’s lifeline to the oil would stay open when it would be so easy for the US from it’s bases in the Phillipines or to a lesser extent the UK in Singaore to cut them off.
No oil = no war.
China as others said, was rich in resources and slave labor. The Japanese expected an easier time as the Communists and Nationalists were too busy fighting each other. The Japanese were able to extend effected control over the coastal cities but never were firmly in control over the country areas. It was a matter of constant warfare to keep what they had taken.