Because no one can tell OP why the interview isn’t checked off. You and OP use vague words such as “a while ago” and “a long time”. Why don’t you tell OP how long exactly it took before your interview was checked off. I provided exact number of days vs vague answers. That is why I say no one can tell you. A long time to you may be a week but to the OP it may be a month. For all we know the OP did the interview in September. You do not understand that your answer in no more helpful by saying “it took a long time”.
Additionally no one can answer for why the school has not checked off the interview other than the school. Again that is why I say no one can answer. It really is that simple. Provide detailed information when asking a question and people can provide guidance but the truth is, if you are questioning what is going on with pieces of your application not appearing, the only people who can answer that is the school.