Grub has size constraints like all boot loaders. USB is non trivial.
I
agree in the longer term that Grub would benefit from native USB keyboard
although EFI firmware may one day make that problem go away (or become
different or something like that 8))
I understand size constraints in the boot sector, but Grub loads other
parts from disk. I would think USB support could be in one of the later
stages from disk.