Most people in this movie don't see what's happening right before their eyes.
Whether it's self-awareness or awareness of danger, they don't want to know about it. AMC's Filmsite notices that "numerous allusions to blindness are sprinkled throughout the film (the farmer's eyes are pecked out, the children play blind man's bluff at the birthday party, the broken glasses of the fleeing schoolchild, etc.), giving the hint that the camera's voyeuristic lens (and its screen-viewing audience) is also being subjected to assault." (Source)
If all that sounds very meta, you're right.
Hitchcock loved to incorporate themes of watching and seeing into his films, implying that we viewers were his accomplices in whatever was happening on the screen. It's a way of getting the audience to engage and react, being forced to look.