Rose Eveleth outlines the reasons:

One is that not being able to tell whether something is human or not can be a deeply unsettling feeling in itself. Artists and directors take advantage of this all the time for dramatic effect. The dread that viewers feel while trying to figure out who is a zombie, or Cylon, or alien might be the very same dread they feel when faced with a very realistic robot.

Another explanation focuses on the disconnect between how realistic something looks, and how well it moves.

There’s always been a lag time between how quickly designers can make things look like people, and how quickly engineers can make them move like us. If a figure that you thought was human started to move jerkily, you would recoil. Similarly, if you were to shake a robot’s hand while expecting a human touch, but instead felt cold rubber, you would be caught of guard. An unexpected break in humanness can be an unpleasant shock, one that sets off fearful and distrustful instincts. “Whenever we see something move, and we’re not familiar with the mechanism of movement, it grabs our attention,” says Andrew Olney, a psychologist at the University of Memphis who works on designing intelligent robots. “If your coffee cup started slowly moving across the table, that would kind of freak you out a bit.”

Finally, a third theory turns to evolution. It suggests that if a robot looks like a human, but moves unnaturally, our brains subconsciously classify what we’re seeing as someone with a disease. This is the same explanation proposed for most feelings of disgust. When we stand near something like faeces, rotting flesh, or a jerking robot, we experience a sudden urge to get away from it so as to avoid catching the infections it may harbour.

Previous Dish on the “uncanny valley” here, here and here.