Because stuttering has little to do with the speech itself but it’s rather psychological thing combined with other factors (biological etc). That’s why speech therapists who put their focus only on the way of speaking (slowing down, deep breath etc) can rarely make progress with patients outside the office. Imagine having a horrible fever and going to the doctor. He starts prescribing you medications and you say “oh btw I’m actually pretty great when I’m alone and have no fever whatsoever when no one’s around.” Do you think that may change the diagnosis and doctor’s approach? I’m not saying “it’s all in our heads” as there can be many factors, but I’m in no doubt that stutter cannot be overcame with motoric exercises alone. What helped me understand the stutter is psychotherapy, book “Redefining stuttering” and by far, the most valuable story - “Speech is a river” (Ruth Mead).