In vulgar American English, "Asia" mostly just refers to the wider Confuciosphere + some parts of Central Asia (though rarely thought about.) Most Americans will look at you funny if you call Pakistan or Jordan Asian, because that's not how we use the word.

> China alone has several distinct cuisines. Why do we act like this is a monolithic concept?

When someone is talking about "Chinese food", they almost certainly are talking about the cuisine established by Chinese immigrants in their country, not food as it exists within China. This isn't unique to China.

More American vulgarism fun facts, "Chinese" wasn't pan-Chinese until somewhat recently. It pretty much exclusively meant Cantonese outside of very specific contexts, like geopolitics. This changed slowly starting in the 1970s, but emphasis on slowly and it still persists in interesting ways today.