Why on the face of the earth....(spoiler)

Pica Condition is characterized by an appetite for substances largely non-nutritive, such as clay, chalk, dirt, or sand.



You want a toe? I can get you a toe, believe me. There are ways, Dude.

I like movies that don't explain everything. Most of Tim's troubles are psychological anyway and eating strange things could explain his anxiety or a physical condition like you mentioned. Either way if I lived in that "bad" environment I would be anxious and act out.