Your optometrist has to get so close because there is only a tiny part of your eye that they can look through (your pupil). It's like trying to see through a keyhole into a closed room.  As you get closer to the keyhole you can see more of what is inside! The optometrist also needs to get very close because it reduces the amount of reflections from your cornea, which would otherwise obscure the view of the other structures of the eye.