Starting in 1926, a warlord, Chiang Kai-shek whose base was in the south of China began a northward military and diplomatic campaign in an effort to unify China under his rule. By 1928, Jiang had managed to bring most of China under his rule--at least nominally. Jiang's power was strong enough to push most of the warlords into making some sort of diplomatic deal, but it was not so strong that Jiang was actually able to eliminate the power of all of the warlords. Therefore, from about 1928 onward, China was ruled by thr political party, GMD."
However, many regions of China, especially those in the more remote inland areas were still ruled by warlords, even though these warlords flew the GMD flag and made a pretense of loyalty to the central government. Chiang Kai-shek knew, however, that his grip on power was tenuous, and that he would have to let these warlords govern as they wanted as long as they remained nominally loyal to the GMD.
The Chinese were quite well aware of the US's "Yellow Peril" fears of China and Japan.
"the vast, faceless, nameless yellow horde the rising tide, indeed, of color " Dower, War, p. 156
But...the United States talked about an "open door" in China so that it could exploit China economically atthe same time.
Japan knew of all this too and turned to the GMD, allowing Manchuria unambiguously to become part of China.
The whole "The enemy of my enemy is my friend" thing.
Chiang Kai-shek used the war against Japan to profit materially and monetarily, dolling out the proceeds to those GMD elites he thought he could persuade to his thinking.
So, there's you atleast three reasons.
1) The uneasy Rule of Chiang Kai-shek to maintain power.
2) The "Yellow Peril" fears of the US against BOTH China and Japan were no secret to China OR Japen.
They were enemies, sure....but the US was BOTH their enemies. The enemy you know as opposes the enemy you don't know.
3) Money went to individuals for profit materially and monetarily instead of being "for the cause" which Japna and China would handle themselves WITHOUT US involvement.