Why every OS still can't resume file transferring? [closed]

"If you are transferring large files over an unreliable network, use rsync with the --partial option. Your use case is rather rare. The share on one system I use has a transfer that takes over a day, with no threat of lost data due to an incomplete transfer (it is slow for some reason to look into, sometime).

Edit:
@Cawas: the more common use case is copying over a file with a new version of the file. If a file has been modified (made bigger in this case), trying to append the extra length of the new file to the old file will result in a corrupt file. Protocols like rsync and ftp can assume you are not doing this."