Wherever lectures happen, regardless of the size of the venue, most of the front row will be empty. Even when the lecture hall is standing room only (should I be so lucky), many of the seats up front will be peopleless. In this photo from a lecture I gave years ago you can see people standing, literally with their backs against the wall, instead of taking a seat in the front.
In Confessions of a Public Speaker, in chapter 4 on how to work a tough room. I explain a technique for balancing rooms. I’d never let an audience stay in the formation shown in the picture above. Instead I’d ask everyone to move forward and close the ranks, forming an actual audience.