1. Stereopsis - different image for both eyes.
2. Accommodation - ability of eye lense to focus on different distances.
3. vergence - eyes rotate slightly inward or outward so that the projection of an image is always in the center of both retinas
Modern 3d technology (movies, VR) provides only sereopsis without accommodation or vergence and vergence-accommodation conflict can cause headaches. You also notice that if you look at different part of the screen from where the movie camera is focused on, the image is blurry.
Conventional lens-screen technology (like Olucus Rift) can't solve these problems. You need something new like light field technology or hologram technology. This is why hundreds of millions if not several billions are needed to develop this technology.