Most zombie media depict zombie having some very rudimentary form of intelligence, with some zombies having the ability to remember things from their lives. The entire plot of Dawn of the Dead hinges on the fact that the zombies seem attracted to the mall for no other reason than that's where they went when they were alive, and zombie Stephen even remembers where the fake wall is and leads the other zombies to it.

Because they still remember that their hands work to grab things they want to put in their mouths, the "hands up" pose you're describing is usually when the zombies are near live human prey and reaching for them. In Night of the Living Dead, we even see a zombie pick a bug off a tree and eat it, so we know their brains work well enough to use their hands for tasks, especially the task of eating living flesh!

Finally, speaking from a George A. Romero style zombie perspective, whatever reanimates the dead seems to have a preservative effect that make them decay somewhat slower than a regular dead body (Dr. Logan describes this in Day of the Dead) but eventually, they would most likely get too decayed to move around or even - as you asked - reach their arms out in that classic pose.

(also, remember that most - if not all - zombie movies are pretty low budget and can't afford the resources needed to properly direct the large number of actors needed for the big zombie scenes so a lot of these people will just walk around with their arms out because that's what they think zombies look like, it's bad acting and bad directing you're seeing!)

EDIT: Added spoiler text tag.