Why are sleepwalkers and people in trances often depicted with outstretched arms? They are shown this way on t.v., in the movies, on comedy shows, etc. Why?
My guess, it is theatrical exaggeration so the audience knows what is going on.
True exaggeration. When we were kids, my sister used to sleepwalk and has a broken nose to show for it…walked into the wall head on…I’m still laughing 45 years later.
Well, Zombies hold their arms out to catch people. Zombies kind of resemble sleep walkers and people in trances, in that they are all kind of out of things, walking around in a daze. Plus, how much easier is it for an actor to walk about with his eyes closed if he holds his arms out in front of him? 
If I can theorize, it’s also a physical state that is instantly recognizable at almost any distance. So a viewer will know instantly that a character is sleepwalking even if they only turn to that channel in the middle of the walk. In short there is no need for an explanation, viewers simply know.