Sorry for the delay in response. I had a baby and have been on maternity leave. You are correct, that the etiology of stuttering is elusive and no one has yet been able to identify an exact cause for speech disfluency. However, if you review the current literature, most experts agree that stuttering is caused by functional neuroanatomical differences in the brain (i.e., white matter tract differences, for example, in the superior longitudinal fasciculus) and the genetic variants that may have attributed to these differences. There are many pathways the brain utilizes to create fluent speech, and the circumstances of the speaker (other auditory input, for instance, a noisy background, different types of speech produced, like whispering, singing, etc.) all play a role in how these pathways converge in the cortical and subcortical regions of the brain to create a motor plan for verbal output. If you are interested in learning more, for a review on the genetic basis of fluency disorders, I recommend has been recommend the paper by Frigerio-Domingues & Drayna (2017) and for information on functional and neuroanatomical differences, Chang, et al (2019).