It is a fairly standard practice in at least some open source communities to add copyright notices to files that people have changed significantly, although there is no well defined minimum threshold for how much permits them to add a copyright notice. Thus, someone else can come along, fork the project, add copyright notices to all of the files and then give the impression that they wrote them, since there is no attribution aside from the one LICENSE file that you wrote. The git history might show the truth, but if they copy the files into a fresh git repository, that metadata will be lost. Projects take files from one another all the time, so there is no guarantee that they will preserve your commit history and then anyone curious who wrote the code needs to do digital archaeology.

That said, file level copyright notices are not perfect (since only the VCS shows who added what lines and that might not be preserved), but it is better than nothing and it is something that is guaranteed to persist as long as people are abiding by licenses. If they are not, that is copyright infringement and the copyright holder can do things like send cease and desist notices in response to the copyright notices being removed.

Also, I must emphasize that I am not a lawyer, but one might argue that it was not willful infringement if someone removed a copyright notice from 1 file by claiming it had been a mistake. However, if they remove it from all files, then nobody is going to believe it was not willful.