Why wouldn’t He? It’s sin, not cooties. Mary’s sinful nature, as being fully human, is not an insurmountable condition for God the Father.
“Mary’s sinful nature, as being fully human, is not an insurmountable condition for God the Father.”
A sinful nature is not insurmountable for God. Providing grace to someone to not have one is also not insurmountable for God. Also, a “sinful nature” is not “being fully human” for our earliest ancestors, Adam and Eve were created with no sin whatsoever but were certainly fully human. What they had is free will. What we have is free will and concupiscence thanks to their choice.
Mary had the same kind of human nature that Adam and Eve did, before they disobeyed God’s law. Fully human, and also sinless.
This is an extremely dangerous error. Sin, or a "sinful nature," never made anyone "fully human". It's basically the lie the serpent told Eve in the garden -- until you sin, you aren't really human.