Why are animals called consumer?

because animals obtain energy by consuming other living things.
Animals are called consumers, or in more scientific terms, heterotrophs (Greek, Hetero = other, troph = food), because we are solely dependent on plants aka producers aka autotrophs (Greek, Auto = self, troph = food) for our nutritional requirements. Plants, specifically green plants containing chlorophyll are capable of making their own food organically from simple, inorganic raw materials, and energy from sunlight, through a process called photosynthesis (Greek, photo = light, synthesis = preparation). Thus, to satisfy their own energy needs, animals, "consume" food that is directly (as in cows, sheep, goat), or indirectly (as in tigers, dogs, cats) sourced from plants.