Because they wanted to be there, or because they could be is the short answer. Manchuria is a different story. They likely went there to protect their supply of raw materials, thinking the Soviets might interrupt those supplies.
Following that, I think there were several factors. They had built a potent force and believed that they could build an empire that they could keep. The relationship between the militarist and Hirohito was a bit of a dog and pony show. They controlled the government because they had a great deal of influence over him, and they in turn "fought for him". With Britain busy fighting in Europe and the US "neutral" they felt that they could consolidate their east Asian holdings. They figured that the US would "blink" and let them keep it, or they would fight for the oil they needed and the US would capitulate from the rigors of a "two ocean war" and let them keep most of what they had conquered. They did not count on the shift in American public opinion or our appetite for war once "provoked" (that provocation is another story of course
).