That's likely because the HDMI Forum don't allow open source HDMI 2.1. [0]
replyThat said, there are signs that it's coming to the AMD drivers. [1] [2]
[0] https://arstechnica.com/gadgets/2024/02/hdmi-forum-to-amd-no...
[1] https://www.techpowerup.com/348723/amd-readies-full-open-sou...
[2] https://www.fosslinux.com/157755/hdmi-2-1-on-linux-complete-...
Seems like a case of under promising and planning to over deliver later. The hardware seems to be 2.1 but they haven't sorted out every feature in the drivers yet.
replyThe DisplayPort 1.4 instead of 2.1 is also interesting. The RX7000 series came with DisplayPort 2.1.
replyDisplayPort 1.4 should be enough to do 4K@120hz. Not enough bandwidth for HDR at the same time though