Except every vendor doesn't. By some magic voodoo witchcraft, x86 remains consistently capable of producing a single kernel that can run on anything from Intel or AMD.
Because they upstream their platform support.
That is literally it. No, it is not a technical impossibility of the ARM world to support common hardware abstractions like IBM-PC platforms. They choose not to.
The "magic voodoo witchcraft" is Microsoft's "Hardware Compatibility Specifications for Windows".
https://docs.microsoft.com/en-us/windows-hardware/design/com...
Linux and the other free OSes run on commodity x86 hardware by targeting the same Microsoft-issued specification. There is no standards body or other neutral organization that defines what an x86 PC is.
It went the other direction: anything new from Intel or AMD had to run the same proprietary, binary-only kernels and applications (since MS-DOS was more of a loader, applications could and did access the hardware directly).
Later, Microsoft started specifying exactly what would be required so that their proprietary, binary-only kernel would run (https://en.wikipedia.org/wiki/PC_System_Design_Guide). It was not the vendors upstreaming their platform support; it was Microsoft dictating what the platform support would be. Even today, Linux's ACPI support mimics Windows', since that's what vendors test against.
ARM systems never had a dominant vendor which could dictate its terms, and new systems were not expected to be backwards-compatible with older kernels.
It's a bit easier to upstream when the gatekeeper sits on the next chair and is paid out of the same pocket as you are.