The reason for this seems simple: It's hard to animate realistic looking people!
But if you think about it a bit deeper, you might realize that some movies did fairly well with realistic animations. Take, for instance, Final Fantasy: The Spirits Within.
Pretty stunning. At a glance, the still image even looks pretty real. But when watching the movie in motion, something seems amiss. The problem is often the characters' glassy eyes. Animators aren't entirely sure how to produce natural eye movement for realistic renderings.
If you can't shake the feeling that you are watching something more like a corpse than a living being in movies like this, you are not alone. You're just experiencing the Uncanny Valley first hand:
The uncanny valley is a hypothesis that when robots and other facsimiles of humans look and act almost like actual humans, it causes a response of revulsion among human observers. The "valley" in question is a dip in a proposed graph of the positivity of human reaction as a function of a robot's lifelikeness.
Interestingly, current technology may be starting to find its way out of the valley. This demo may surprise you. The person in the video, named Emily, isn't actually real. But she sure looks it. If you didn't know she was animated, you'd never think twice, and never get that uneasy feeling. At least, I sure didn't. I'm not sure if it's a good thing that we may no longer be able to distinguish between real and fake, given the problems that could arise in court cases and such. Only time will tell.