Why didn't Japan declare war on Russia. Germany declared war on the United States after Pearl Harbor, so it would just seem logical that Japan would support Germany by declaring war on Russia.
OK, it is time to look at time because the **** did have designs on Siberia.
1) 1939 and the Japanese Kwantung Army of Manchuria had a serious go at the Soviets in the summer at the battle of Khalkhin Gol. They suffer a crushing defeat (45000 dead?) at the hands of an unknown corps commander named Zhukov! Enthusiasm for invading undeveloped, unpopulated Siberia wanes a bit.
2)1941, June 22 and Hitler invades Russia, but doesn't invite Japan to join the party - in fact doesn't even tell them! Throughout the war, there is little successful cooperation among the Axis except the supply of DB601 and 605 aeroengines to Italy.
3) 1941, July. Roosevelt and all Allies embargo oil shipments to Japan which has less than 1 year in reserve for all purposes.
By December, the IJN is down to a 3 month supply.
So, do you go for the spruce trees, snow and nomads of Mongolia and Siberia against a proven tough enemy or the OIL, rubber, tin and rice etc. of weak/defeated Europeans like the Dutch, French and British...no brainer!
Hitler's declaration of war on the USA is so crazy. The American thirst for revenge after Pearl Harbour was so strong that they might have left the Nazis alone for some time in favour of crushing Japan ASAP.