In no other situation do so many humans have to keep quiet, watch the speaker attentively, and keep their minds on the message without an opportunity to respond.
The responsibility on both sides is taxing; hence, the very act of speaking in public breaks the natural rules of human discourse and is thus unnatural.
The result of this unnatural act is to make both the speaker and the audience somewhat nervous in their new roles as dominant speaker and submissive audience.
" Sweeping" the audience with one's attention —— looking at all sections of the audience at one time or another and regularly —— helps the audience keep its attention focused on the speaker.