The USSR was never an “ally” to the US troops dying in the Pacific, until shortly before the end of the war when they invaded Manchuria. While Americans were dying at Pearl Harbor, Tarawa, Guadalcanal - the USSR observed a truce with Japan.
We did the same with some German allies in WW2. We should have invaded Finland by that logic, but we didn’t.
Also worth mentioning that over and above any political concerns, the Soviets basically didn’t have a navy worth the name (for multiple reasons, as the British found out when they got their Lend-Lease battleship back from them after the war) so they *couldn’t* carry the war to the Japanese at sea. The Red Air Force was mostly set up to be a short ranged, tactical support force and couldn’t have contributed to the Pacific island hopping campaign if they’d wanted to. Finally, the Red Army in the East was lacking in equipment, troops and most importantly leaders if they had wanted to go after the Japanese and Manchukuo troops on the mainland. Even the pre-war Soviet victory at Khalkin Gol against vastly outnumbered Japanese troops was only accomplished by a gifted commander (by Soviet standards, anyway) who later could not be spared from the German front and even then was only accomplished at huge cost in men and materiel. Of which neither could be spared post-Operation-Barbarossa until very late in the war when it was obvious that Germany was about to collapse and there wasn’t going to be another Battle Of The Bulge style resurgence.
Post war research has shown that it wasn’t that Stalin didn’t *want* to go expand in Asia, it was that he believed he couldn’t muster the men and equipment to do so.