So the first thing to note is that at the period in question, the resource of Siberia were not fully developed. Even if Japan had routed the Soviets completely, and taken control of everything east of Lake Baikal, Japan would have to invest significant resources into creating the extraction and processing infrastructure needed to fully exploit those resources. Meanwhile, the European colonies in Asia already have most of that infrastructure established. Assuming it can be captured intact, resource exploitation can begin almost immediately, rather than at the end of a long and potentially fraught economic development plan.

Now, granted, there were significant factions within Imperial Japan pressing for war with the Soviet Union. Espescially within the Imperial Japanese Army, the Soviet Union was seen as the existential threat to Japan. Not only was the IJA virulently anti-communist, but the IJA feared potential Russian revanchism for the Russo-Japanese War, espescially given Japan's new major holdings in Manchuria which gave these two a long and ill defined land frontier. Indeed, the IJA had substantial plans for a war against the Soviet Union, however many of these fell from favor in the aftermath of the Nomohan Incident, also known as the Battle of Khalkin Gol. Border conflicts had sparked up sporadically between the Japanese and Soviets along their shared border, with some going each way. However, Nomohan saw a Japanese division soundly defeated by Soviet forces under Zhukov. Importantly, the defeat highlighted how much further ahead the Soviets were in the realm of logistics and mechanization, when compared to the Japanese. For the IJA, this seemed to render the prospect of war with the Soviet Union far more daunting, and helped to redirect attention towards the south, where--as Germany overran the home territories of the European colonial powers--the European colonies seemed far more vulnerable.

Finally, although certainly not least, there is the issue of China. Moreso than any other factor, in the lead up to World War II, nearly every aspect of Japanese foreign policy led back to the war in China. The situation had devolved into a political quagmire that Japan had no real escape from. The Army's escalations had failed to defeat the Nationalists, despite Japanese occupation of many key cities. Yet, those same escalations had seen Japan commit a tremendous amount of blood and treasure into the China War, and now the Army (and the population as a whole) wanted major concessions from China in order to justify this 'sacrifice'. At the same time, Japanese operations in China had been souring relations with the United States for an extended period of time. There was significant pressure to try and find a diplomatic solution, but the Japanese demands for concessions in China could not be reconciled with American insistence of China's sovereignty.There's more to elucidate here, espescially on prior Soviet aid to the Nationalist government, and the gradual termination of that relationship, but I'm not well read enough on that particular area to go into it.

We can see an example of this when Japan chose to occupy French Indochina. The Japanese goal here was to try and cut off land access to China, in order to finally bring the war there to an end. Yet it was this escalation that led to the American oil and scrap embargo that put a huge ticking clock into the Japanese mindset. Without oil and other sources of raw materials, Japan's war machine would grind to a halt relatively quickly, and Japan would not be able to launch a war even if it wanted to. From Tokyo's perspective, with every passing day, Japan grew weaker, while the US grew stronger. The Japanese mindset was one of "use it or lose it". If Japan was going to go to war, it would have to do so now.

At which point, the resources of South East Asia, and the few remaining transport links between China and the outside world drew Japan's attention south. That the IJA had received a painful defeat in relatively recent combat against the Soviets no doubt further tilted the scales in favor of a strike south.

I hope this serves to answer your question. If you have any follow ups, please feel free to ask.