Why do all the Chinese people in American movies speak Cantonese?

Because most of the immigrants in the United States or those who have obtained American citizenship in the United States in the early years were those who did business in the coastal areas of Fujian and Guangdong in China in the early years. For example, most Chinatowns in the United States are from coastal areas of China, so many local Chinese or overseas Chinese who enter the American film and television industry still speak Cantonese.