Why can the US be regarded as a collage?

At the most begining, America was regarded as a melting pot(大熔炉).That's because a lot of immigrants went to America and their own cultures mixed together and a new culture formed among the immigrants, that's, they no longer obey to their oringinal cultrues but to the newly formed cultrue.
Later, another immigrants came to America with their cultures and their cultures were mixed together and formed a new culture which they obeyed to, however they did not abandon their oringinal cultures and they still obey to the oringinal ones. For example, We Chinese went to America, our culture mixed with the cultures of the other immigrants cultures and we obey to the mixed cultures. However, we still use chopsticks, we spend out traditional festivals such as spring festival, dragon-boat festival and so on .
So during this time, America is called a salad plate(沙拉盘), which means that the essences of all cultures are remain the same, just covered by the newly formed culture--the