To do that, you'd also have to harden everything - including the hard drive's firmware - so you get the proper boot sector.
Not all bios bugs can be worked around in firmware -- if the memory controller isn't initialized correctly, all processing is suspect. And if the manufacturer can't fix a bios, then it falls on the shoulders of the OS writers -- who do not have neither the motivation to fix 100's of board-specific problems. Worse yet, if I wanted to inject malware, I would pretend to be a motherboard manufacturer and submit bad kernel code to beleaguered OS people.
Not all bios bugs can be worked around in firmware -- if the memory controller isn't initialized correctly, all processing is suspect. And if the manufacturer can't fix a bios, then it falls on the shoulders of the OS writers -- who do not have neither the motivation to fix 100's of board-specific problems. Worse yet, if I wanted to inject malware, I would pretend to be a motherboard manufacturer and submit bad kernel code to beleaguered OS people.