A couple reasons, many of which have been pointed out by others already.
But a big one I find is that this question answers itself: Smart people are smart because they are quiet.
Intelligence requires spending time in meditation, analyzing things, and linking them together, which requires organizing the focus from the outside in. Focus can be disrupted by speaking. Idealism is closely related to intelligence, which makes intelligent people avoid mistakes as much as possible, resulting in less speaking and more listening
Being quiet means you give yourself the opportunity to shut out your own voice and listen to what others are saying. Knowledge sets the foundation for action, and actions display intelligence. You cannot apply intelligence — or be “smart” — without adequate knowledge.
And I don’t just mean intricate knowledge that requires years to consolidate. Just listening to what people say instantly shows you the type of person they are, and entails basic requirements for communication such as their sense of humor, their names, and their preferences.
Allowing others to speak is a basic practice that is crucial for development.
I know it’s a little forward, but this quote sums it up quite well:
News Hub Creator firstname.lastname@example.org