The answer is simply because they're using a single display. From there they are just moving the lenes and using software IDP adjustment. This works for the most part - but you are limited on how long the display is and this effect could lower your FoV as well. Going to a single screen is the main reason for the limited IDP adjustment. Simple as that. Dual can still allow an IDP of 100+ (think weird face/eye placements) and work just fine.
Fresnel lenses work in the center of the circle on the lens.
If you go too far out from the center you begin to see the fresnel rings on the lens.
You can fix this by making the gap between the fresnel lens and screen smaller, but this means a larger fresnel lens size.
Having a larger lens means a thicker lens too to make the picture come into focus.
So take that size and weight, take a minimum strength of the target audience
if you take eye glasses and tilt them and read print through the glasses, if you look through the glasses section nearest what print your reading this is the larger size. now read the same print put holding your glasses the same spot but moving your head, so the eye reads the print out from the glasses lens farthest from the print the print is smaller.
the largest print is when the glasses lens is held normally on the eye this is largest.
so by rotating the lens you affect the focal power of the glasses lens.
likewise a vr headset that has a tilting eye glasses lens would have varifocal ability.
