Webcams record faster than 60 Hz. Sound perturbs the recorded image every scanline, not just every frame. The techniques that reconstruct audio from video do it by looking not frame by frame, but line by line. 60 Hz times 720 vertical resolution is 43200 Hz and way more than enough data.
Because webcams record at 60hz (max), they can only capture enough data to reconstruct sound at 30hz, way below the human voice range.