Because the element of deterrence is more effective than the element of surprise.
http://gunfacts.info/pdfs/gun-facts/5.1/gun-facts-5.1-screen.pdf
Page 31:
All the "element of surprise" does is allow you to shoot a criminal after they have already begun to attact you (and I still retain that ability when open carrying). I would rather deter that criminal from ever choosing me as a target to begin with. Why would a criminal pick a person who they can see is carrying a gun when they can wait two minutes for that person to leave, or go down the street one block and have their choice of many, many targets that do not appear to be armed? That just makes no sense at all. Criminals aren't THAT dumb. They have 3 goals: to get what they want in the easiest manner possible, to not get shot, to not get caught. Messing with a visibly armed civilian does not promote ANY of those three goals, ESPECIALLY when there is such a large percentage of targets available that are NOT visibly carrying a gun.
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That's funny. I would assume they were intelligent enough to recognize the deterrent value of a visible display of the capability of using defensive force making the effort required by the criminal to overcome the target to be much greater than the benefit gained should the criminal be able to overcome the target's defenses.
And, while we're here, let's go ahead and debunk the myth that the open carrier will get shot first in a robbery:
Gun Owner Saves Lives In The Richmond VA Golden Market Shooting
To the OP,
If you want real answers from experienced open carriers, I would suggest you check the Michigan forum at:
OpenCarry.org - Discussion Forum