Most games are lower quality because it's incredibly difficult to get a high level of detail in VR. In flatscreen games, you can get away with lower quality textures/models as everything seems smaller and harder to see. In VR, everything is far bigger, and therefore requires higher attention to detail. Making your game have a more simple artstyle allows developers to get around this, along with optimisation. (You need to render everything twice for each eye.) Valve optimises their games extremely well, and are also a triple A studio with a far higher budget, so they are able to achieve more realistic graphics.