I've thought about this before, actually. I have two guesses:
It's because everything in a video game looks animated. The lighting and shading isn't totally realistic, so the entire scene takes on a cartoon-ish appearance. We don't see animated characters in an animated world as being out of place, but we see animated characters in a real world being out of place. A humanoid robot is a lot like an animated character--it has fake skin, eyes, and mouth, and so it looks just a little freaky. With animation, the appearance of falseness is there, but it's obscured by the animated environment.
Video game developers only have to concern themselves with making flesh and other organs that look realistic. All they do is create the shape of the object and texture it. They have freedom to play around with the apparent properties of the object, its textures, and its movement. On the other hand, if you're building a robot, you need something that looks like skin, stretches like skin, moves like skin, has sensors, is resistant to heat, is resistant to water, is cheap to produce, and so on. There are only so many materials available to us in the real world, and not one of them can
perfectly imitate human skin.
Can you make a realistic looking human as a computer model? Yeah. But you can't do it in real life, unless you have perfect materials to use and can place all the little motors you need that act as facial muscles. Not to mention things like the mouth, the nose, the eyes, and so on. On a computer, you just draw the shape in 3D software, texture it in Photoshop, and code its animations. In real life, how are you going to imitate a human mouth? You need something that stretches, something to control its stretch, something hard, white, and shiny like teeth, some way to keep the area wet and slightly reflective... and so on.
It all comes down to this. Robotics is hard. Computer animation is easy.
