I'm going to guess because it's easier than fitting a plug into your spine like in The Matrix.
Let's be honest, if that (or the squishy things in Existenz) were on the table, a hell of a lot of us would be all over them.
For myself, I bought a Vive. It gets less use than I'd like, but DCS World is awesome in it. Just being able to look over your shoulder, or being able to peer down from the edge of the cockpit rather than having your POV swivel around a totally fixed point is absolutely great. Plug in the old Thrustmaster and you don't need to worry about hand tracking.
It really is very good for some things.