It's caused by the panel/lens hardware. Center resolution is laptop-like, but by 35ish degrees off center that's down by ~half.[1]
[1] line graph in https://kguttag.com/2023/08/09/apple-vision-pro-part-5b-more... . General caveat that Guttag's "not possible"s sometimes have an implicit "if you're not doing anything weirdly off-VR-mainstream". Monitor replacement has very different constraints than mainstream gaming - like, refresh rates (and thus bandwidths) of 60 Hz and lower (even 20 Hz) can be fine, while for VR games that'd be absurd.