The Soviet Union did take official control of the territories they were promised under the Molotov-Ribbentrop Pact, which happened to be a part of Poland (now parts of Belarus, Ukraine, and Lithuania). They also took Kaliningrad Oblast from Germany (parts of East Prussia).

They'd already taken a large chunk of Polish land. Poland wasn't really the main enemy here, it was Germany; shifting Poland's borders westwards would make the new Communist German area weaker.

Poland itself had been ravaged by war and many Polish partisans had fought against the USSR (after all, the USSR invaded their country along with the Nazi Germans). Annexing the country would have put all these partisans inside the USSR where they then form a major obstacle.

Finally, remember that the sovereignty of Poland was quite literally the spark for the beginning of the European Theatre of World War 2. The Soviet Union didn't want to antagonise France and Britain too much.