Why do we need to grow plants to make food?
We are part of a biological system, and we evolved to be part of it. Plants are the base of that system, and the main means of capturing energy and inorganic compounds and converting them to nutrients which are bio-available, along with bacteria.  The further we move from the niche we evolved to occupy, and the further we change the environment of the rest of the system, the more likely we are to bring about our own extinction.
Edit: This answer is somewhat opinion based. We haven't finished the experiment that proves this thesis, yet.