That's surprising.
China has 4 times as many people.
But, the USA has 8 times the per capita income.
Plus, at first glance, home delivery in China looks like it would be more complex, slower, and more expensive than in America, but I don't know how to find reliable Chinese data on that.
My instincts say that Alibaba owes a large measure of its success to the absence of competitors which, most likely, is vigorously enforced by the Communist Party.
Alibaba probably makes a lot of money through foreign sales.