This is funny, because I posted the identical question (this has always baffled me) a few months ago when we discussed this subject. Translated, the answer is: Cuz' that's our story and we're stickin' to it! Because they have to no matter what evidence is shown, they don't think they can afford to be shown in error on anything like this. It's bunk dressed up in fancy clothes and given the name "Holy Tradition".
It's propped up because there are a lot of Catholics out there that worship Mary that would no longer call themselves Catholic if this were removed. It is a tool. More Catholics in SouthAmerica believe in "the virgin" than the Lord. Is that not what we read a few threads ago. And have we not seen the parades, all of us, in south america as in other parts of the world where an Idol of Mary is paraded around the streets of these small towns in large processions Celebrating the Goddess who Gave birth to a God named Jesus. They lavish her and cover her in money.. The modern Catholic 'miracles' always have something to do with Mary.. The weeping virgin that cries water or blood, the virgin idol with images in her eyes or some such nonsense, the wierd (and I'm afraid, not understood even by me) crucified virgin...