Probably because people that have stammered for their entire life have a large neurological component involved in their brain as to why they stammer. So for many, stammering is their ‘normal’ way of talking, and fluency has to be induced and maintained via speech therapy and learning/using techniques. I for one, can speak very fluently using my techniques and have made a huge amount of progress in the way I speak since I started therapy almost 18 months ago,to the extent that to the average listener they wouldn’t be able to usually tell I stammer, however if I stopped using my techniques and control methods when I speak, I would go back to stammering a lot more