Yeah cause , lol.
Soviet was a super power with an actual tank dev and building industry same goes for their Mili plane. China being closed to , surprisingly , both west and east for a good while leading them getting a lot of tech and with a large country and man power. They can easily develop their own tech.
And they export their arms so lol ?
Not to mention their tech tree actually get boosted by Taiwan since they get their tech from the west.
Same can be said for the US but the thing is NATO nation in game tend to have their own stuff along with some more from US themself.