Why do we still need to eat?

I know that sounds silly, but hear me out.
Our bodies don't exactly use the food itself, just the nutrients inside-- right? So with all of the nutrient, protein, fatty, and juice drinks they have today ... Well, why do we still need to eat? We obviously get enough nutrients from the drinks we consume.
We haven't talked about this much in my class, besides I'm in Animal Biology.

Thanks everyone.