ANSWER:
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Because animals don't have chloroplasts in their cells like plants do. Plants use these things to make food and energy through a process called photosynthesis. Photosynthesis takes place in the thyroidal membranes of the chloroplasts. Also, If animals would have ability to capture sunlight and produce energy for themselves there would be no need for plants and carbon cycle would not exist. There is a very important relationship between animals and plants creating a cycle of energy and carbon. That's why also many vitamins cannot be synthesized by animals.
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