I have a separate boot. I'm certainly not the only one; there is still running hardware out there where the BIOS can only access a small fraction of the disk. This was made worse by the explosion in disk sizes over the ten years. If you don't use a separate /boot, you need to get lucky and hope that your GRUB1.x stage 1.5 (and 2?) are within the range BIOS can directly access.
As for the design picked by the bootloader: The filesystem drivers are modular. I expect that, if you wanted to, you could fairly trivially write a GRUB-specific filesystem driver. Heck, it could have a hardcoded table of files, with a hardcoded set of offsets at which to find them. Or it could look for a table of such values stored in a block of records at a known place...but now we're on our way to reinventing FAT.
I think you're confusing the full capability set of the Linux Kernel with the relatively meager capabilities required to understand filesystems.