That would be an insanely bad move. Parts of China had been under hostile rule for decades or longer, and the revolution succeeded at all due to the nationalist movement. Merging with the USSR would feel like subjugation, it would take the victory against Japan, and the victory over the Manchu dominated Qing, and just throw it out as far as the majority of people would be concerned.

Mao and Stalin also differ greatly on the National Question and Mao was cognizant of the fact that there was no merging without being told to deport a hell of a lot of people. Heck Chinese in the Soviet Union had been deported almost entirely in the late 1920s-early 1930s. Including Chinese volunteers who fought for the Bolsheviks and along side the deportations of Korean volunteers because the Soviets said Japan could use their existence as pretext for expanding an ethnic or linguistic border.

They would be on edge waiting for orders to start coming saying to create more homogenized regions, and the removal and relocation of nationalities deemed "unloyal" which would also not be good for the Soviets because Xinjiang already had been Soviet aligned and during that process in the early 20s Central Asian rebels fled there and caused fears of incursions.

Plus the Soviets wanted a buffer. There is a reason they repeatedly rejected Mongolia's requests to become an SSR.