As I mentioned in my first reply, the firmware is expected to have complete knowledge of the address mapping of the system, and the OS isn't.
There are lots of ways for the firmware to convey information about the address map to the OS, but it may not always be complete. Mapping two devices to the same address is a recipe for trouble.
That's not to say an OS can't change these things. PCI/PCIe hotplug rely on the OS to set the BAR address for newly connected devices. And that may require rearranging existing device mappings so that things fit. And so, OSes that are capable of hotplug can often do address reallocation for all the devices, but firmware would need to have allocated the devices used to boot the OS already, and you may as well use those if they're there, right?
There are lots of ways for the firmware to convey information about the address map to the OS, but it may not always be complete. Mapping two devices to the same address is a recipe for trouble.
That's not to say an OS can't change these things. PCI/PCIe hotplug rely on the OS to set the BAR address for newly connected devices. And that may require rearranging existing device mappings so that things fit. And so, OSes that are capable of hotplug can often do address reallocation for all the devices, but firmware would need to have allocated the devices used to boot the OS already, and you may as well use those if they're there, right?