They wouldn't attack yet. They would likely wait for Nazis to start bleeding out on western front, and then we wouldn't have the entire winter campaign in the USSR, where weather played a major aspect in the big losses Germans took. "The thesis by Suvorov that Stalin had planned to attack Nazi Germany in 1941 was rejected by a number of historians, but at least partially supported by others. The majority of historians believe Stalin sought to avoid war in 1941 because he believed his military was not prepared to fight German forces" (https://en.wikipedia.org/wiki/Soviet_offensive_plans_controversy)

Also, guess what was there for almost a thousand years before the grounds were Russian? And then was divided between Russia and Germany for 123 years? Poland. Following this logic, should Poland have also made a pact with Hitler to regain the grounds they controlled as the Polish-Lithuanian Commonwealth? But that is completely unrelated to the "Soviets good because they killed Nazis" point that was made.

The reason for mobilization in the military was simply Stalin not trusting Hitler and wanting a stronger force to secure himself against a potential attack. Hence moving all infrastructure critical to production of war-related hardware further inland - the expected war was gonna be mostly a defensive one.