Because Microsoft decided so. Probably long, long ago already, in the days of DOS 1.0.
A rename doesn't modify the file. So it makes SOME sense to keep saying it isn't modified.
But it sets the archive flag. That certainly makes sense to get it on the backup with its new name.
after ren *.dll *.bak
why does the *.bak file not have todays date?
XP Pro + SP2