I worked with one of the HR people from one such places, if they were correct, it is mostly the quota and budget. Basically, they get a specific budget each year and they need to spend it, so they post old vacancies to with new shine not only on their own sites, but also on various commercial places(Xing/StepStone/Indeed/Linkedin/Monster etc.) to burn that budget and hold interviews to show that they are trying to fill their hiring quota for the year. It is just a fake practice, because if the budget remains unused at the end of the year, then it'll be reduced next year and if they did not perform enough posting and interviews, then personnel in hiring department will not get promoted or will get bad reviews due to low quota coverage.
Not sure how real it is, but it can be related or one face of the story.