if you don't say anything, the default assumption is you don't know anything
This is an US-ism. In northern Europe it is often the opposite: constant talking heads are regarded as empty and being concise is a valued skill. We're just nice to the talking heads and let them fill the air with words, but we don't regard them as better than the quiet ones.
I've often experienced that the longer the quiet persons don't say anything, the more nervous the talking heads get, until they can't restrain themselves and demands that the silent ones say what they think.