But it does???
History disagrees with you. The Soviet Union and the PRC inherited the contradictions of Tsarist Russia and the Qing Dynasty, and you can't just handwave that away. Russia was one of the eight nations in the Eight-Nation Alliance that came to oppress China during the Century of Humiliation. Russia also had unequal treaties with China. To make a long story short, Lenin was cool about annulling those treaties while Stalin was less so. Meanwhile, Khrushchev threatened to nuke China. Russia should be paying China reparations for its role in the Century of Humiliation if anything. The relationship isn't equal because Russia owns China for historical wrongs and ought to repay China in the form of reparations. "Uh aktually we're no longer Russia we're the Soviet Union." Yeah, and look what that got you.