Why would the Soviets ever strip forces from the Far East and leave themselves vulnerable to Japan? Even against Germany, when their survival was in doubt, they left sufficient forces in the Far East.
Absolutely. Let's look at the official numbers:
06/22/1941: 703,714 people, with 10,080 guns and mortars (50-mm mortars are not counted), 3188 tanks and self-propelled guns (All light tanks; there were 132 SPG only on May 9, 1945), 4,140 combat aircraft, 94 ships (Warships of the main classes).
12/01/1941: 1,343,307 people, with 8,777 guns and mortars, 2,124 tanks and self-propelled guns, 3,193 combat aircraft, 96 ships.
07/01/1942: 1,440,012 people, with 11,759 guns and mortars, 2,589 tanks and self-propelled guns, 3,178 combat aircraft, 107 ships.
11/19/1942: 1,296,822 people, with 12,728 guns and mortars, 2,526 tanks and self-propelled guns, 3,357 combat aircraft, 98 ships.
07/01/1943: 1,156,961 people, with 13,843 guns and mortars, 2,367 tanks and self-propelled guns, 3,949 combat aircraft, 101 ships.
01/01/1944: 1,102,991 people, with 16,827 guns and mortars, 2,009 tanks and self-propelled guns, 4,006 combat aircraft, 102 ships.
05/09/1945: 1,185,058 people, with 20,695 guns and mortars, 2,338 tanks and self-propelled guns, 4,314 combat aircraft, 93 ships.
The Far East was seriously fortified, the experience of June 22, 1941 was seriously taken into account here. In the USSR, they were not stupid and always seriously counted on the threat from Japan.
But all the same the main reason why Japan did not move to the USSR was the Fighting on Khalkhin Gol. While in the USSR it was considered a local border conflict, Japan considered it a serious conflict at the level of the Russo-Japanese War.