The HDMI standard isn’t directly built into USB-C or Thunderbolt. You need some layer of active conversion, and I’m not sure how many dongles there are out there that would be able to do that for such a high bandwidth spec. If they’re out there, are they actually doing it well and supporting all of HDMI 2.1’s features? I don’t know…
I was afraid Apple would leave this out, but because of how big of a refresh this was supposed to be, I didn’t really think they would. If it’s a bandwidth issue like some are speculating, it’s a bit unfortunate since laptops with dedicated Nvidia GPU’s are able to have HDMI 2.1 by connecting the port directly to the GPU.