shouldn't the machine just be able to load your OS kernel directly?

Well this horse might already be out of the stable with UEFI but creating a way to make variables available to the kernel image doesn't seem impossible, maybe stored as EFI variables in a standard way, or as files on the boot filesystem. How did HPUX boot? That's what EFI was designed for. Maybe the tools aren't there now because there wasn't enough participation by Linux vendors or the Linux Foundation when these issues were decided, no one is going to hand you a seat at the table for these kinds of things, you have to insert yourself worth force if you don't want to be run over.