Why was Michael Myers credited as "The Shape" in the first 2 Halloween films?

My best answer comes from an answer that was given to this same question in 2006:

Adult Myers is introduced in the screenplay as a shape jumping on the back of Marion's car. Therefore, for script consistency, the term was used for the rest of the draft and eventually the series.

Also, there is further explanation and links to the script here.

The Shape is what John Carpenter referred to Michael Myers as in the script. Throughout the movie you only see the outline of Michael lurking in the shadows, non descript and very much a "shape".

http://www.pitofhorror.com/newdesign/halloween/aboutmichael.html

In the actual script, the first usage of "the shape" is used to describe the mask Myers wears...

"Darkness, with a small shape in the center of the screen. As MAIN TITLES CONTINUE OVER, CAMERA SLOWLY MOVES IN on the shape.

We get closer and closer until we see that the shape is a Halloween mask. It is a large, full-head platex rubber mask, not a monster or ghoul, but the pale, neutral features of a man weirdly distorted by the rubber. ."

http://www.dailyscript.com/scripts/halloween.html

Subsequently, the mask (originally designed by Don Post (later becomming Don Post Studios)) was referred to as "The Shape"

EDIT: It should be noted, much to my dismay, that while watching the movie with the director's commentary it was revealed that the mask is actually a halloween mask of my boyhood idol, Captain James T. Kirk of the Starship Enterprise, painted white. Ugh. My image of him is forever ruined...

this is just my guess but I assumed The Shape was a symbolic reference from Dr Loomis talking about years of trying to reach Micheal Myers "..for I realized whatever was living behind that boys eyes was purely and simply evil!" Dr Loomis saw the truth no one else did about Micheal Myers. What was inside him wasnt human. He wasnt human he was just..evil Pure evil that had taken the shape of a man.

When ever Lorri sees Michael stalking her from a distance he is seen by her as just a shape, she doesn’t know who he is or what he even looks like he’s just a stagnant figure appearing every now & then. In the newest movie Lorri even calls him the shape because she seen his reflection in a mirror but like tradition he disappears from her sights again. I believe the true reason for calling him the shape is simply because Lorri when sees him stalking her he is “The Shape” & other things may even trick her into seeing the shape but it really isn’t Michael like when she chopped some dudes head off by mistake- oh wait that never happened never mind.

Well in the first film I think Michael was played by two or three people if I remember a behind the sceans interview but I think the shape sounds more scary and dehumanises him as none could surly be that evil and th e aim was to scare us the viewers and not much is known abiut Michael at first what drives him to kill his backstory is unknown to later unlike Freddy who is out for revenge as he was set on fire so we know about him and Jason we know his story yet I don't think Jason knows he is doing wrong and was once a good person but Michael all we know at that point is he killed his sister and in the remake her boyfriend and his deadbeat father but of course later as we learn more about Michael he is no longer called the shape So wish Dr Loomis books were real I'd love to read them