I sometimes think it is because they are so bad at expressing themselves verbally that writers take to pen and paper in the first place.
There might be something in this. I don’t know about you, but I find it much easier to construct a coherent and sensible thought if I’m sat at my desk with either my laptop or pen and paper than if I have neither. Then the thought just goes round and round my brain and reaches no conclusion.
On the other hand, I can “write” what I think to be perfect passages in my head while walking along and then be unable to commit them to paper because they have already evaporated – even if there are mere moments between thought and attempt to capture. They are but flutterbyes on a flower: here, and then gone; seen only for a moment.
Oh, and I hate public speaking, but here I am quite happily publishing to the world…