I suspect it's because Chinese carriers want to maximize used of Chinese airspace and minimize use of foreign airspace. This may have to do either with national security interests (the idea that flying over home land is somehow safer than over foreign land), or with air navigation fees (pay no fees or yuan at home, have to pay hard foreign currency to Russia). You will notice that just about all such Chinese flights, and all those that you mention, pass over ARGUK, which is a waypoint on the China-Russia border at the most northeastern point of China. By going over ARGUK they maximize use of Chinese airspace and minimize use of foreign airspace.
To UA, that may not matter. China or Russia are both equally foreign to UA. In terms of air navigation fees, UA pays both in home currency (USD) anyway. So they would just pick whichever route is more favorable. UA flights typically do not go over ARGUK.