Animals absolutely do synthesize nutritious substances that you can't get out of plants. For instance humans, as long as we ingest the 9 essential amino acids,

Animals absolutely do synthesize nutritious substances that you can't get out of plants. For instance humans, as long as we ingest the 9 essential amino acids, can create from scratch any protein our body needs. Our bodies are also capable of turning protein and fat into carbohydrates when needed (https://en.wikipedia.org/wiki/Gluconeogenesis https://en.wikipedia.org/wiki/Gluconeogenesis). Animals, us included, are not inefficient filters, or tanks that hold minerals and energy. We are chemistry sets designed for survival in a wide range of conditions.