Why? As I pointed out, the Soviets typically kept about 50 divisions in the Far East throughout the war. The Japanese at their height had about 75 there and for much of the war, far less. Of these, only about 10 or 15 could have been used in an offensive in any case as the rest are engaged in internal garrison of China or in fighting the Chinese Nationalists or Communists. The Soviets began the war with equal or better equipment to the Japanese and far more armor.
They kept a substancial advantage in armor and artillery right through the war (yes, most of the armor were older models like the BT-7 and such but these were substancially better than anything the Japanese had). The Soviets also built much heavier fortifications and made very liberal use of mines; two things the Japanese eschewed form much of the war as defensive and defeatist...not the Bushido way so to speak.
About the only useful thing Japan might have managed out of an attack on the Soviets was conquest of Sakalin Island and its substancial oil reserves.