I think it comes down to Cyan trying to push really nice visuals, while originally coming from a background in pre-rendered graphics. The original Myst games made use of pre-rendered viewpoints and bits of full-motion video to enable graphics far beyond anything realtime game engines were capable of at the time. When they decided to transition to realtime graphics, they wanted to maintain as much of that visual fidelity as possible, and as a result, their games tend to be rather demanding on hardware. This was true even back in 2000, when they launched the original RealMyst, which brought Myst into a 3D game engine, but ran poorly on most computers at the time, and it's kind of been a trend they've had since.
And at least in Obduction's case, they're back down to a relatively small development team, while making a game that has visuals comparable or better than what you might find from much larger teams. Unfortunately, that apparently comes at the cost of optimization, although the game's slow pace means than getting only around 30 fps should still be plenty playable, at least on a monitor. That doesn't really work as well in VR though, where 90+ fps is ideal.
And no, it's not just AMD hardware. Even some with high-end Intel i7 processors and $500+ Nvidia GTX 1080 cards are complaining about the performance in VR. I'm sure there must be room for performance improvements though, so things will likely get better, at least to some degree.