This is just not good reasoning. Just because some things have been mechanistically explained in the past 50 years doesn't mean that any arbitrary condition will eventually be non-somatically explained. In fact, the argument (vacuous as it is) could go the other way - anything that has left after we've shrunk the pool so considerably is considerably more likely to not have a non-somatic mechanistic explanation than anything in the original pool.

And FWIW, CBT is effective for IBS. But with the gut-brain connection, we really are pushing up against the bounds of mind-body dualism with stuff like this.