I think it's because this was obviously lip-syncing over the recorded version (no cords on the guitars!) and the producers, or McGuinn, didn't want a close up during the lead instrumental because it was improvisation to begin with and there would then be a dead giveaway mismatch of playing to the...
From 1:00 to about 1:20 you can see the beam move behind the trees to the field at right at the ground level. Therefore a static arc light would be ruled out, unless it was mounted to a vehicle traveling on a distant road near the fields.
I think it's because this was obviously lip-syncing over the recorded version (no cords on the guitars!) and the producers, or McGuinn, didn't want a close up during the lead instrumental because it was improvisation to begin with and there would then be a dead giveaway mismatch of playing to the...