Socialism

Because the state uses violence against them to suppress them when they try to wield undue influence. That's the answer. It's like why can't Russian billionaires just walk into the US and buy the FBI out of their way? They can't do that because the state uses violence against them to prevent them doing that. The US state does not use violence against its own bourgeoisie, against the trans-Atlantacist bourgeoisie who say get away with raping kids and get a slap on the wrist, who do Epstein stuff (separate case from aforementioned believe it or not), who engage in open corruption "lobbying" within the US. Who through their means exert influence directly on the government.

This isn't to say China is perfect. Xi Jinping purged a lot of corrupt officials who were letting billionaires buy their way to breaking the law, to being exceptional, etc. A lot of officials were arrested and even executed. The fact is they'll keep trying but China has antibodies. It has an ideologically communist military, an ideologically communist justice system, and ideologically communist state law enforcement and intelligence apparatus that serve the party. (Conversely the US also has countermeasures such as courts, laws written for bourgeoisie, intelligence and law enforcement, etc that ultimately restrain the proletariat)

It gets back to the nature of the dictatorship of the state. In the US you have the dictatorship of the bourgeoisie. The state exists to protect their interests and carry out their will and to oppress and control the proletariat. It doesn't perfectly carry out the will of the bourgeoisie, it can sometimes fail to oppress the proletariat or let them get in a win here and there. In China it's a dictatorship of the proletariat but it's also as in the case with the inverse not perfect, it broadly serves the interests of the communist party guiding the proletariat but there are times where it may not adequately restrain billionaires or adequately uphold the interests of the proletariat.

It is the nature of things. Spotting a less than perfect record doesn't mean much as no system is perfect. We must ask instead what it primarily does and how it is organized rather than nit-picking at a failure here and there. Otherwise we have confusion about the intent of the US DotB in it letting workers get a win here or there in this time or that.