why was the US military production critical to Allied success in WWII?

Answer 1

Because the United States vastly outproduced the Axis powers. Not only did the US equip itself but played an important role in equiping various other allied powers including the Soviet Union and Britain. To take one example, the United States could build merchant ships at a rate faster than German and Japanese ships could sink them.
One point though, "American" should be "North American" since after 1941 war production in the United States and Canada was closely integrated.

Answer 2

The US huge production output overwhelmed the Axis powers. We could lose 10 planes to each Axis plane and still be ahead. During the Battle of the Bulge German troops saw thousands of tons of supplies in depots and knew they had a tiny fraction in comparision. Many of the German troops knew the war was lost at that point.

Answer 3

Industrial production in both theaters of war was virtually shut down. Factories were destroyed and raw components were non existent. Without American contributions on land, sea and air both efforts would have been lost.

Answer 4

our win kinda sucked as it was done with superior numbers and superior mass production. Like when the empire sent out tons of cheap tie fighters but the Rebels took them out 10 at a time with x-wings.

We were making cheap tie fighters and losing lots of them.