""The biochemical composition of hair makes it impossible for digestive juices in the stomach to break it down. It therefore accumulates, much like it builds up in the plughole of a bath or shower, attracting more hair and ..."
The rest is poorly formatted but this should serve as an explanation.
You are what you eat
“Keratins are a family of fibrous structural proteins; tough and insoluble, they form the hard but nonmineralized structures found in reptiles, birds, amphibians and mammals. They are rivaled as biological materials in toughness only by chitin.”
It is insoluble...