And why is that?
According to Catholic teaching, Mary's body was assumed into Heaven upon her death.
Before 1950, the assumption of Mary was open to Theological debate, but once the Pope defined it as doctrine it is considered a closed topic in the Roman Catholic Church.
Outside the RCC, it's still open to debate. Personallly, I think all of the focus on Mary is a bit silly, although I will light a pink candle for her soon.
Even from a cynical point of view, if the Christians from, say, 330 AD to 1530 AD had thought it was even remotely possible for there to exist a physical relic of Mary, some entrepreneurial bone-merchant would have come up with a hank of hair or a vertebra. But no. Not a scrap. Nada.
From the day when the Christians emerged from the catacombs, nobody believed that Christ left his mother's body rotting in the earth.
Even if you don't believe that, you certainly shouldn't say the body doesn't count. We all believe in the resurrection of the body.
And why is that?
According to Catholic doctrine and the traditions of the Catholic Church, the Blessed Virgin Mary (Mary, the mother of Jesus) "having completed the course of her earthly life, was assumed body and soul into heavenly glory."[1] This means that Mary was transported into Heaven with her body and soul united. The feast day recognizing Mary's passage into Heaven is celebrated as The Feast of the Assumption of the Blessed Virgin Mary by Roman Catholics. This doctrine was dogmatically and infallibly defined by Pope Pius XII on 1 November 1950 in his Apostolic Constitution Munificentissimus Deus.