Well for starters Japan was not rich in natural resources, so expansion into the East Indies and china was necessary for an aggressive Japan's survival. The Japanese army leadership had long ago decided that China would need to be subjugated. It also didn't help that many Japanese army and government ministers were war hawks who wanted war, there constant aggression help to sour relations an make war inevitable.

A prime example would be Col.mutaguchi Renya the commander of the Japanese forces near the Marco Polo bridge. When a small skirmish broke out between his troops and Chinese ones he sent in reinforcements and escalated the fighting. Tensions rose even more when reports came in that Chiang kai shek was sending reinforcements to the area. The Japanese assumed the engagement would be a quick one and they would be able to overtake the important areas of china.

So back to your original question as to why the Japanese invaded china despite having Manchuria. They invaded because many of the top army officials wanted war, the Japanese needed as wanted the economically important areas of china, and they assumed it would be an easy conquest.

Sources: Japan's imperial army it's rise and fall by: Edward drea

The Second World War by: John Keegan

The history of modern china: Jonathan fenby