That's likely because the HDMI Forum don't allow open source HDMI 2.1. [0]

That 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-...