Why didn't Japan stab the Soviet Union in the back during the Soviet-German war in WWII, instead of going to war against the US?

This question was already being proposed at the time, when Japan had not yet declared war on the US. The Soviet-German war at that time was the period of the German army's devastating advance, and there were two voices in Japan at that time, one was to go north, that is, to stab the Soviets in the back, as you said. The second was to go south and invade Southeast Asia. The Indian Ocean was the seat of British and American interests. This would inevitably offend the United States, and Japan was fully aware of this. It was difficult to distinguish between the two voices for a while. But the war situation at that time swayed the decision. At that time, the Soviet Union was ill-prepared, and one said it was deliberately defeated, anyway, the Soviet army was losing ground at that time. Japan felt that there was no need to help the allies any more, and there was no need to occupy Siberia at that time when no oil fields had been discovered, and Japan was a resource-poor country, while Southeast Asia had abundant resources for expansion. So, in the end, the voices going south won out over those going north.

Then came the Japanese sneak attack on Pearl Harbor.

That's how history is set, and history can't be hypothesized, if it can be hypothesized at all. It is certain, then, that at the very least, there would have been no Soviet Union in the Allied forces, and history would have been rewritten.

No, the spheres of influence had already been divided at the time of the Triple Alliance. The entire Soviet Union and European America went to Germany, Italy was responsible for Africa, and Japan, the Asia-Pacific.