They thought about it. The way the Japanese government saw it, they had two options. One was to invade the USSR and start a war in the north. The other was to start a war with the United States, in the south and east. They didn't think they could do both at once. There were some who advocated for the former and some who advocated for the latter.

The Germans invaded the USSR on June 22, 1941, and on June 24 the Japanese Army and Navy agreed that for the time being they would focus on their southern/eastern front, not get involved against the USSR. They also resolved, however, to make preparations so that if, at a later date, they wanted to get involved with the German-Soviet war, they could quickly do so. It was what Hasegawa calls a "wait and see" policy — see if the Germans quickly overran the Soviets, and if they did, quickly snatch up the prime Soviet real estate in the east.

Why hesitate? The Soviets and Japanese had signed a neutrality pact in 1941, but they had border clashes previously. In 1939 the Soviets were victorious in an onslaught at Khalkin Gol/Nomonhan, which Hasegawa credits for sobering the Japanese army with regards to any illusions about the ease of taking on the massive Red Army.

Instead they pursued their eastern/southern strategy, which culminated in the bombing of Pearl Harbor that December. While the idea of potentially attacking the USSR stayed in their minds, it never became a great time to start in on that after that point — the situation was never quite ripe from the Japanese perspective. It is an interesting piece of historical speculation to imagine what might have happened if instead the Japanese attacked the USSR when it was weakest, and put off any explicit hostility against the USA until later.

On this issue, I have found Tsuyoshi Hasegawa, Racing the Enemy, chapter 1 ("Triangle Relations and the Pacific War") very useful.

This also has to do with the huge landmasses that needed to.be occupied. Japan at the time was seizing Chinese land but never took a large controlled and connected area on the chinese east coast. So a conquering of a nation as large as Russia, would stretch their power very far. And besides, the east of russia had very little political or economic benefits as opposed to more readily available and smaller territories.