Of course their are people who are afraid or can't express themselves publicly, it's called stage freight. This does not mean that they are not intelligent or smart people, but they just have that fear of facing the public. I am also an example, I used to be sacred of speaking in public, especially during my university days, but later I was able to overcome it, by trying even when I knew I have thus phobia. I think.its something that is common with quiet and shy people, I am talking from experience, because I am.the quiet type when I was growing up.
You overcome your fear by facing it, rather than avoiding it... And so like every other social anxiety disorder, stage freight is something that can be overcome.. And the best way to overcome it is for the individual in question to deliberately and gradually put him/herself in those situations where they would have to face the crowd and speak in front of them. One could start with maybe 3 person I.e speaking to a little crowd of like 3-6 persons, before finally increasing the numbers until it reached a bigger number and so on like that...I have a school mate like that back then who also suffered from this stage freight, though his own wasn't that severe coz at least he can face his class and even do some explanation while facing them...He only becomes very nervous when he is facing the whole school...Instead of him avoiding that situation, he continued regardless until he was able to overcome that fear and today he feature in local news and public speaking...I so much believe in learning and that no one was even born into perfection, but with learning and the grits to continue regardless, then individual can grow, learn until they achieve perfection...