Mary had to give birth to Christ, and have no other children before or after. Christ had to die and rise again, John had to baptise, Elizabeth to give birth to him, Moses to lead Israel out of Egypt and so on. And it had to be these particular persons, not others - do you think God had a spare prophet handy in the time of Moses or John? Or a second Immaculate virgin tucked away in case Mary didn’t feel like fulfilling the vocation he had planned for her?
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Remember that he preserved Mary from the moment of her conception from all sin, original and actual, knowing precisely what her task would be. He told Joseph to take Mary for his wife, and didn’t leave Mary to fend for herself as a single mother (even though she probably had male relatives who could serve as guardians for the child) or find another man for her husband as she possibly could have done.
He told Abraham that the child who would breed a nation from him would come from Sarah, and not from Hagar. He gave Pharoah and his servants some pretty detailed information in the dreams that Joseph interpreted, telling them exactly how long the famine would last and so on. Christ foretold that Peter would deny Him three times, and in Acts God sent a vision to Paul to tell him to go to Macedonia. So it would appear that God certainly has at least some of the detail of our lives sorted.