Why can't animals use nitrogen in the air?

Because the nitrogen in the air is in a form not usable to animals and plants. The only way animals get nitrogen to build protein and nucleic acid is by eating it. This is usually through plants, which get there nitrogen from the soil. They get it from the soil cuz bacteria in the soil turn the atmospheric nitrogen into a usable form. In a water ecosystem cyanobacteria a.k.a. blue-green algae transform the nitrogen from the atmosphere into usable forms of nitrate