Traditionally, a person changes into a wolf during the full moon. Depending on the specific mythos, that could be up to three nights in a row out of every 28. Sometimes, if the moon goes behind a cloud, they change back to human, until the moon comes out again. This is usually used for comic effect.
Why so many movies? Why not? How many vampire movies have there been? How many alien? how many robot? They make a move because they think once they release it, everyone will want to see it and they'll get lots of money!
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The European folklore of the werewolf, which is a curse that causes the inflicted to change into a malevolent beast during the full moon, has fascinated both those who tell and those who enjoy a good scary story for centuries! As such, it's a natural theme of many supernatural, often stupid and sometimes comedic stories, some put to film. If it's well done, it sells. I think one of the best of breed was "An American Werewolf in London."
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The idea of a werewolf feeds on that essential fear that a man has a savage inside just waiting to be released. I can't think of a good werewolf movie in recent years. Dog Soldiers maybe (I haven't seen it all the way through)?
Zombie movies take this to a macro scale, the fear that society is on the edge of collapse, feeding on those it used to feed. There's been a lot more activity on the Zombie front in recent years: Zombieland, Shawn of the Dead, Day of the Dead (and other Romero properties), 28 Days Later (and sequels) among others.