American composers born in the early 1900s, who came of age in the Roaring Twenties or during the Great Depression, were exposed to the music of Gershwin, Copland and Roy Harris while still young.
American classical music already existed for them. It was big, and wore its heart on its sleeve. Sometimes it could swing. It was tonal but unafraid of dissonance or modulation. This was their received music base.