Why don't antibodies generally bind to food and drugs?
B cells containing receptors against food's or drug's molecular structures may exists. However if the naive B cell binds the molecule without a second signal coming simultaneously from CD4 Tfh cells (linked recognition) the B cells recognizing this antigen will become anergic or will die by apoptosis. This occurs with antigens to which we are chronically exposed, like food.

Reference: Janeway's Immunobiology, 8th edition. Chapter 8 (The development and survival of lymphocytes). In particular section 8-26, p320-321.