2 things: budget and hardware limitations
In addition, a lot of these criticisms are against games developed for HTC Vive. Unlike the Rift, the Vive was only announced relatively recently. Developers haven't had years of lead time to create high fidelity, long games for it. I just started Chronos, and it looks better than anything I've played on Vive, except maybe Call of the Starseed (but that's been in development for years, but only has 2 hours of gameplay).
In addition, the Vive developers tend to be very small indy groups. Often one developer (Vanishing Realms). Without a big art department, you won't be getting a photo-realistic experience.
Given time, as VR matures, and developers have more time to develop, we can expect better looking games.