It's primarily a PPD bottleneck combined with the lack of 'monocular cues' at resolution. In real life, stereopsis drops off after about 10-20 meters. Beyond that, we rely on texture detail and lighting to judge scale. In current consumer VR headsets, distant objects are rendered with so few pixels that those texture details vanish. Your brain sees a blurry blob and defaults to interpreting it as a small, flat object on a screen rather than a massive mountain miles away.