Because she would have been too old to fit the description of the female in the hebrew prophecy.  What part about that is confusing?

And you can go on about Mary and Joseph not having sexual relations.  I fail to see how that matters in the case of Roy Moore who fondled a 14 year old girl and had a 14 old girl touch his own genitals.  If anything, that would not make this like Mary and Joseph at all.