“Those who know do not speak. Those who speak do not know.”~ Lao Tzu, Tao Teh Ching
Lao Tzu talked about confidence. People who are confident in their knowledge and expertise don't have to speak out to prove themselves. They confidently believe in themselves. Plus, there may be trade secrets they want to hold for themselves, thus why share?
On the other hand, those who have less credentials are eager to speak out. When they don't know much, they want to demonstrate that they do know something. They are seeking for approval and acceptance.