To create animated hair, you basically have to create each strand of hair and use physical forces like gravity, wind and collisions to simulate its movement. As you might expect, this draws rather heavely on computer resources. So hair is one great compromise between what you want and what you can afford in terms of computer time. And then there are all the parameters: how much should it bend ? Does it curl ? How heavy is it ? How sticky ?

Below is a very early version of Santa's beard. While it isn't all that bad, it does not really look like real hair.