Why was Germany, being better prepared for war than her opponents, was defeated?

This is due to two key factors: 1. industrial capacity; 2. human resources. Germany’s industrial capacity was significantly inferior to that of its opponents. In addition, on the side of the opponents of the Germans there was a numerical superiority. Thus, Germany was only able to maintain a kind of plan for the first three years, and then the industrial power of the USSR and the United States simply crushed the German troops.