Y U No

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Rigor Mortis... :D

How fast do you think you would move if you were half dead??

For suspense.... if they were faster, there would be no suspense, they'd get to the living.... and thus the biggest reason is there'd be no movie or TV show

do you want to die quickly?

They're not so slow in the movie I Am Legend.

because zombies have no brain witch means no nerve to connect with the the spine so the can only limp

Because they are deciding whether or not to eat you or just have a nap