What's so strange about that? There are alot of NT figures we don't know where the bones are.
Well, first, she was the Mother of Our Lord. Further, she was entrusted to the care of St. John, who we know lived to a rather ripe old age and served as mentor for St. Polycarp. Hence, one would expect her grave/tomb would have been known to the early Church. This is somewhat different than the fate of many of the Apostles who scattered to spread the gospel. St. Thomas, for example, is believed to have travelled as far as India and it is my understanding the communities he founded there exist to this day. In these instances it is understandable we wouldn't know of their resting places since distance and poor communications would hinder any sense of certainty. (Though now that I think about it, I think there is a place which tradition holds is St. Thomas' resting place. I'd have to check.)
Similarly, tradition gives approximate resting places for Peter and Paul and while I'm sure you don't believe it, I'm fairly convinced Peter's remains are precisely where Tradition has always held them to be, i.e. right under St. Peter's Basillica in Rome.
So, yes, I do find it strange. Though I'm not surprised you don't.