Question

Why are plant and not humans known as producers even though they cook their food ?

Answer 1

In the context of Science, the term 'producers' refers to organisms that produce their own food. Plants fall under this category, for they produce their own food in the true sense, manufacturing it from sunlight, water and Carbon Dioxide through the process called Photosynthesis, which uses a pigment called Chlorophyll. 
Now, even though humans cook their own food, 'cooking' is not equivalent to 'producing' food. We still have to obtain raw food- vegetables, etc and modify (heat, etc) the same to obtain our food. We cannot manufacture our own food using no natural material (even man-made foods use veggies, meats, etc). This makes us heterotrophs, or consumers, that rely on others for food.