China still buys jet engines from Russia, they’ve had 35 years to create a fighter jet engine and they still can’t figure it out. Not everyone can solve complex issues.
Maybe they don't see the point of wasting manpower when they can let Russia handle it. (seems like one of Russia's historical fortes)
Russia is about to become a completely, wildly different place. The Putin's regime is on its last legs, and the next leader, would very possibly distance himself from China.