We did, until Churchill suckered FDR into Lend-Lease and violating the terms of neutrality.
America was very reluctant to get involved in another European war in 1939, largely due to the isolationist sentiment stemming from the Great Depression and the memories of World War I. The public hugely favored non-involvement in foreign conflicts, believing it was unwise to engage in wars that did not directly threaten the United States. A 1939 poll indicated that 94% of Americans opposed entering the war.
Among others. But FDR and his cadre of Commies (like Harry Hopkins) wanted to take the pressure off Stalin after Hitler launched Operation Barbarossa in June 1941, you see.