Japan could expand in two directions: west and northwest or south. First they wanted to go west (China) and northwest (Siberia). China wasn't a problem, but going northwest was. The soviet army was much better than the Japanese expected. They were soundly defeated in the battles of Khalkin Gol. One of the commanders on the USSR side was Georgy Zhukov.
Those battles were extended border clashes, but more than enough to show the Japanese army it wouldn't be a walk over. So they stopped there and then. One of the reasons likely was that Siberia had a lot of minerals, but you need to extract them. That's pretty hard, and would take many years.
As to the Axis treaty: it was a very nominal treaty. Just about nothing was done in cooperation. All they shared were some signatures on paper. There was no coordination, conflicting interests and very little else. The Japanese attack on Pearl Harbour was as much as a surprise to Hitler as it was to Roosevelt. A bigger surprise for Hitler, because the Americans could read the Japanese code.
Going south was much more profitable for the Japanese. The oilfields in the Dutch Indies were huge and in production. Malaysian and Thai rubber were for the taking. All they had to do was knock out the US fleet in Pearl Harbor.
Yamamoto warned against it, but not so wise heads prevailed. With hindsight you can win any war, this is a clear example. It's even worse for the Axis. Richard Sorge was a Soviet spy in Japan. He could conclusively prove to Stalin that Japan wasn't going to attack in Siberia, and he could safely withdraw the Siberian troops and use them for the defense of Moscow. That was even worse than not cooperating and not supporting each other.