It is hard to be silently brilliant: lots of thoughts occur when you open your mouth.
But not all thoughts that occur are brilliant... not in a person who isn't opening his mouth... 1 or who are saying the same things over and over again.
Things not worth thinking are also things not worth saying but you don't know that until you hear the echo. 2
What happens when you speak?
Humans didn't quite become humans until they had something to say. But speaking doesn't make someone human: having something worth to say does.