Just because they develope their own chinese-based open-source ecosystem doesn't mean they have to cut themselves off from the english-based one. But by using the english-based one, they are cutting themselves off of their potential native chinese-based one.
My point is that it would be easier for chinese people to develope in their own language. I'd consider it silly if I had to learn chinese or greek just to program "hello world".
> because most people can see the value of being able to share code globally, which means using English.
You could share code globally even if the chinese code in mandarin. You can translate code from chinese to english and vice versa. It would be even simpler in assembly.
Also, there would be more code overall if programming didn't have an added hurdle of having to learn english in the first place. If people around the world could code in their native language, we'd have more programmers and more code.
Using your argument, the chinese should write their novels in english too. But everyone would consider that absurd. That's the same for programming. It's an absurd system that exists, but hopefully things will change in the future.