People simply buy too much of the "Japan has no resources, so Japan must conquer" narrative. In reality the Japanese resource exploitation was not that different from the late European colonization, and the South Manchuria Railway Company was pretty much a Japanese East/West India Company in China.
Before the Washington Naval Treaty, Japan almost went the 8-8 fleet program (8 battleships, 8 battlecruisers), and except the last 4 battlecruisers, all the rest were either completed, in construction, or had their budget allocated by the time of the signing of the treaty. That a total of 421,358 tons of standard displacement of capital ships when Japan didn't control Manchuria. Their maintenance would be financially crippling, but there's no material problem in constructing them. In comparison, Yamato's standard displacement was 65,000 tons.
Also Japanese full-on invasion of SEA only happened in late 1941 right before/after Pearl Harbor. If Japan was in dire need of resources French Indochina and Dutch East Indies would be occupied far earlier, but in reality Japan only took over Tonkin, the northmost part of French Indochina to block a supply route for China, out of the whole SEA until late 1941. This pretty much shows that "secure supply of oil & other resources in SEA" was less important than "blockading China" until late 1941.
As for why targeting the USSR, it's simple. USSR was seen as an expansionist power and it's right on Japan's footsteps, and it's considered a threat that might try to attack Japan, either out of ideology, revanchism or just because. It's the same reason why Japan maintained presence in the intervention of Russian Civil War even after the Whites have collapsed and all other powers withdrawn. It's not like there's any profit in the form of cash, resources or pride, but that the communists were perceived as the utmost threat to Japanese security. This hadn't changed in the view of many but that faction was purged in 1936.