Because a substantial minority of people have functional gastrointestinal issues that are well addressed by the same treatments as somatic pain: antidepressants, stress reduction, and placebo treatment.

The rate of actual gluten intolerance is some 4-5 fold less than the rate of functional disorders - it's nigh on impossible that all those eating a gluten-free diet have gluten intolerance (let alone celiac), but it's extremely likely that they have functional gastrointestinal disorders.