We all know HDD's have firmware
Every device does.. if it didn't, it would not know what to do (Well, not as much as devices now days do), while you can flash firmware (Like I put a custom one on my DVD-RW, to make it region free) it's very risky, and to be honest, it's safer to just leave alone if it's not broken.
If your having problems, fair enough that's a good reason to flash, if not, then no real reason tbh.
I'm sure if you started getting problems, you could contact manufacture and they would provide new firmware (If they had one) :-)
BIOS, generally get updated to support new hardware (Like a new CPU that just came out for example), so their needed. (After-all, wouldn't you be angry if you got a new CPU and it didn't work due to BIOS?) HDD's generally don't need that sort of thing.
So really, that's the reason for lack of HDD firmwares avaliable on company sites.