Cuz I've never seen a realistic CGI shark, which IS perplexing cuz sharks only have so many joints and limited movements.. But they always look fake and horrible.. Any organic subject - such as a human or animal or anything "alive" made from organic material, is very difficult to model (3D object model) with the correct joints and range of motion.. and its equally difficult to animate said model to look realistic.. for example, when a human or animal walks, more body parts are moving than legs, so to look realistic, all those other body parts must move correctly and realistically..

So, huge strides have been made in human animation, due to the black suits with the white balls on them that tracks movement of all joints and appendages.. but you can't really do a suit like that on an animal and have that animal act out the exact movements needed for the film, so the result is hand animated and looks fake.. Its VERY difficult to do..

---

Friendship is like peeing your pants.. Everybody can see it, but only you can feel the warmth..