Kinda to end the speculation on this (well maybe, this is still just an idea), I wanna point out what Lampa said in the other thread that's currently up about this.
"Looking at Valhalla's page on Amazon, it seems that PS4 and PS5 version are separate SKU, and contents on the disc might be different, where as Xbox has one SKU for both consoles, they probably don't want to bottleneck themselves with non-UHD Blu-Rays because of base Xbox One"
This is the reason, most likely. It's because MS are pushing for one unified packaging...so you'd have to have a non-UHD Bluray to account for the old ass base Xbone, and then put a UHD version in there as well so Series X users wouldn't go crazy about a huge additional download. PS4/PS5 having different SKUs solves this, and so for Xbox why increase the packaging cost when you could just shave it down instead? The one box "Xbox" family packaging is the likely cause here.