There are a lot of reasons for that.
The world has been dominated by the Anglo empire for centuries. For most of that time, China was weak and backward. Of course, people would be migrating to the metropole and not some broken, impoverished empire.
Secondly, America is a land of immigrants. Immigrants built this country. It has also been very rich for many generations, thus its reputation is solidified in the global imagination. China has only recently become developed so in the minds of many people, it is still a poor and backward country even though in many ways, it is more developed than the West in certain areas nowadays.
Eventually, there will be more and more foreigners coming to live in China than there are now as it continues to rise.