The problem is that it is easy to get the real disk with GPT/MBR of e. g. /dev/sda1 but it is harder to get the real disks (it can be multiple!) of e. g. /dev/md1.
And on restore it might be even more complicated e.g. when there are now 4 drives instead of 2 on backup etc. Raid config is very dynamic.
File backup has nothing (or at least not much) in common with image backup.