Well you asked why wasn't the US allied to Germany in the war. Asking that makes it sound like you weren't aware that Germany declared war on the US, because that's a pretty obvious reason for the US not to be allied to Germany.
The US was allied with the "Allies" before the US become formally involved in the war. They were already providing material aid to Britain/France before Pearl Harbor / war declared by Germany. I'm definitely not the one who is "unaware" of the key details in this discussion.
The US provided support to the side they ideologically agreed with -- it wasn't just a case of "Germany randomly declares war on the US".
