Why does food need to be basic to be digested?

Answer 1

it doesn't. it can be either basic or acidic.

Answer 2

You question could have two meanings; the food is simple, or the food is chemically basic.

Food doesn't have to be chemically basic. A lot of your food is acidic; e.g. oranges and other fruit, soda. Your body has buffers which resist changes in pH, and all the stuff go to the stomach which produces a decently strong hydrochloric acid anyway.

Food doesn't have to be simple either; your body can digest almost everything you eat, with the notable exception being a couple of polysaccharides which can't be digested, such as cellulose (this is dietary fiber).