Also, you need to understand that different parts of the brain are activated when actually speaking to an observer, person, or if you are involved in a social situation. Take therapy for example, different parts of the brain are active depending how you are receiving therapy. If you get it over the phone or on video call your brain can tell the difference and less parts of it are activated. I used to think and question my stuttering similarly. It's ridiculous we stutter rarely if not ever when completely alone. Once you add an observer though we react in a totally different way. Language and sociality are integral to our species, they make us human in the first place. Verbal language and complex communication defines our very species and makes our genus unique. Our evolution was directly influenced by our ability to share complex/abstract thoughts through speech. There was no hunting or gathering as a group without language. If one tiny part of this complex and poorly understood part of our brains goes awry or is off it will be apparent. The fact stuttering is just as common in any race is proof it has affected our spcies since the beginning of our messy evolution. I am sure stuttering even appears in some way with hunter gatherer groups which takes away mental health from the equation for why we stutter. No cases of anxiety or depression in hunter gatherer egalitarian groups has ever been observed. I think of it as maybe we have an issue with too much stimulation when around others and in how our nuerology handles social interaction. We know how to speak and can be fluent on our own. It almost defiently is something in this regard. Perhaps our brains use, lack, or have different nueral pathways say a fluent speaker posesses.