A couple of technical reasons might be: ensuring that you can "grep" for any 4-digit year and find applicable files; or, that a "diff" makes it more obvious what is being added.
I prefer ranges personally. Also, I believe only the year of first publication is technically required anyway.
Not sure about US law, but in Australia, Copyright notices are not required at all, they're granted automatically under the law and only serve as a reminder -- so it's irrelevant.
I prefer ranges personally. Also, I believe only the year of first publication is technically required anyway.