alt Hacker NewsIt's because it's only 8mm cubic volume.
But you can get etched glass scale encoders that will give you 1um accuracy for basically nothing these days (IE <50 bucks).
You'd still be better off just using cheap linear motors with encoders built into the rail though.
Encoders are cheap, even interferometric setup is possible to do at perhaps <$300 if you happen to live near the NeHe rust belt. Actuator backlash and stiction will still be a problem. Good actuators are expensive. Solving backlash is what is nice about this setup.