If we're talking the classic MS-DOS experience on a PC that most people mean when they're talking about DOS on PCs, there's more to it than just x86. It's also the whole legacy of "IBM PC Compatible" from the BIOS to the memory map to the way it interacts with peripherals and the applications it can run. After all, there were a handful of non-IBM PC Comptaible MS-DOS machines that, while they ran DOS, wouldn't run the same things.
Really there's two different use cases for something like FreeDOS - running legacy code and using it as a minimal environment for things like firmware flashers and such. There's nothing keeping FreeDOS from keeping up to date with the times and continuing to boot on the latest and greatest 64-bit only, EFI only machines and staying in its niche as a minimal OS for specialized/embedded use cases. On the other hand, running legacy code will need all that compatibility stuff to still be around - x86_64 CPUs that still support Real Mode, legacy BIOS compatibility modes, various memory map and interface setups that date back to the beginning, etc. FreeDOS updated to run on an entirely different x86_64 arch that doesn't actually run anything legacy is not really any different to, say, porting it to ARM or RISC-V or whatever since functionally it's basically completely different anyway. I would expect heavier-weight OSes to have compatibility layers and handlers within its HAL for most stuff, but for low level bare metal OSes like DOS that sort of defeats the purpose of a slim, direct OS like it's aiming to be.