U.S. simply participated in WWI for too short a time, only 1.5 years. It didn't produce much stuff in WWII by spring 1943 either - everything was in desperately short supply then. If it wasn't, why didn't Normandy invasion happen a year earlier?

And lack of preparedness in the previous years before WWI was deliberate, because public was too much against the idea of interfering into European affairs and politicians made every efforts to make it as physically difficult as possible. Zimmermann Telegram changed that.