Our military prevents them from dealing with domestic issues to the extent they want to (Taiwan, Hong Kong). If it were pre-occupied with fighting a civil war China wouldn't have to worry about it as much. Even puppets have to maintain the illusion of independence, and say "re-unifying" with Taiwan would be a step too far for even a puppet President.

Beyond that, as long as we're strong they still play second fiddle. Of course over time if they keep selling us stuff to build up their economy and stealing our technology to build up their military they will catch up, but if we fall off our pedestal they don't have to climb quite as much as they would otherwise.