I think part of getting by with a lower PPD is the IRL pixels are fixed and have hard boundaries that OS affordances have co-evolved with.

(pixel alignment via lots of rectangular things - windows, buttons; text rendering w/ that in mind; "pixel perfect" historical design philosophy)

The VR PPD is in arbitrary orientations which will lead to more aliasing. MacOS kinda killed their low-dpi experience via bad aliasing as they moved to the hi-dpi regime. Now we have svg-like rendering instead of screen-pixel-aligned baked rasterized UIs.