At the time of the United States entering the war the Germans were at the gates of Moscow, for the US to not focus first on European theatre of war risked Russia being knocked out of the war or, equally bad and a constant fear for the Allies for most of the war, Stalin making a separate peace with Hitler.
Also, the British and Americans had agreed previously that in the event of war with both Germany and Japan that they would focus on Europe first, for the Americans to have gone back on this would have meant a major loss of confidence between them.
You’d have had much weaker cohesion between the Allied powers, with significant consequences to the conduct of the war.
And it took more than four times as much shipping to move a division from the US to Australia as it did to move the same division to Britain or North Africa.