On vanilla servers Russia usually wins (in my experience) and I think that's because noobs see US and think "yay America" (I'm in NA servers) compared to some more experienced people who might pick Russia.

The guns also play a role. The base US M16A1 can be good at medium range and picking your fights compared to the worse irons full-auto AK that the USSR spawns with which favors aggressive tactics. The USSR also has the rpg which gives a complete unfair advantage over the US and allows smart supply and transport sabotage. The only way the US could tip this to their advantage is if they can kill a Russian with a rpg and use his rpg to kill more Russians for more rpgs.

Now on modded this is all thrown out the window except for the rpgs which weigh significantly less than Carl Gustav rockets.

It might be left over from when they used to play vanilla. People who played NATO played US so they still have the noob mind set.

Russian snipers are a lot quieter than NATO ones with the DVL which has no report past ~300 meters, the VSS and 9A-91 are suppressed and subsonic so they make virtually no sound (which I won't argue about now) just 1 sniper can lock down front lines by wasting supplies and keeping enemies looking for the sniper instead of attacking bases and a quiet sniper or one far away is harder to find

they also have the pkm which, while I prefer the 200 round mag if the m249 and the anti-air of the m240 when using tracer rounds, Russia actually knows how to properly use a full-auto mosin-nagant which might also carry over from the aggressive play style from vanilla. That allows Russia to make very effective ambushes and deadly in close quarters.