That's actually not true. I believe with the latest generation of Nvidia cards (10 series) they made rendering multiple similar viewpoints dirt cheap by tweaking the hardware and the rendering pipeline.
From the arstechnica review on the 1060: "GPU Boost 3.0, Fast Sync, HDR, VR Works Audio, Ansel, and preemption make a return too , as well as the ability to render multiple viewpoints in a single render-pass."
From my limited understanding, VR is difficult because of the tolerances required. For regular gaming, slight frame drops were annoying, but didn't break the experience. Thus, it was reasonable to ship a game that was able to hit 60fps 99% of the time, and just write off the remaining 1% of the time. For VR, not only do we need to hit at least 75fps, the tolerance for frame drops is much much lower (a stutter while you're watching a monitor is annoying, the same stutter in VR could make you lose your balance). To aim to hit 75fps and guarantee that you'll hit that 99.9%, 99.99%, or 99.999% of the time is where the difficulty lies. I'm sure most of the HN audience has experience with just how difficult it is to tack on an additional 9.
From the arstechnica review on the 1060: "GPU Boost 3.0, Fast Sync, HDR, VR Works Audio, Ansel, and preemption make a return too , as well as the ability to render multiple viewpoints in a single render-pass."
https://arstechnica.com/gadgets/2016/07/nvidia-gtx-1060-revi...
From my limited understanding, VR is difficult because of the tolerances required. For regular gaming, slight frame drops were annoying, but didn't break the experience. Thus, it was reasonable to ship a game that was able to hit 60fps 99% of the time, and just write off the remaining 1% of the time. For VR, not only do we need to hit at least 75fps, the tolerance for frame drops is much much lower (a stutter while you're watching a monitor is annoying, the same stutter in VR could make you lose your balance). To aim to hit 75fps and guarantee that you'll hit that 99.9%, 99.99%, or 99.999% of the time is where the difficulty lies. I'm sure most of the HN audience has experience with just how difficult it is to tack on an additional 9.