A significant number of the interfaces in this list are used for booting. If you "simplify" your firmware design by hardcoding to any of these you will make users of the others unhappy.
Granted, you can imagine dream and modular firmware code that can be selectively trimmed down on a per-platform basis. EFI might get there but why would you do this for (expensive) PC motherboards? A development effort to lose some of your customers?!
By the way USB + managed flash is among the most expensive solutions; this matters for low cost embedded systems.