The reason is that portraying it as the world vs R is exactly what Putin wants.
Putin wants to frame this as a second "Great Patriotic war" for the motherland
This reminds me of 1920 -- the newly resurrected Polish army helped the Ukrainian army to capture Kyiv. The Bolsheviks then framed it as "The Poles are trying to conquer Russia again, like in 1610" and they got many White Russians to join the Red Army.
Don't play into Putin's hands. This war is Putin's war, not Russia's
So who is driving the Russian tanks, flying the planes and firing the artillery?
Very good point.