Why did St. Augustine and St. Thomas endorse ensoulment at 40 or 80 days?

I know the story that they lacked our current biological knowledge. I know the story that they still considered abortion to be a grave evil. These two excuses seem inadequate to me. Regardless, they are still quoted to this very day by pro-abortion apologists. Aquinas was even quoted by name in *Roe vs. Wade* in support of historical theological confusion about abortion.*
*cf. *The Fake and Deceptive Science Behind Roe v. Wade: Settled Law v. Settled Science* by Thomas Hilgers, W., MD . Since they were so brilliant all they had to do was look at Psalm 51:5 “in sin did my mother conceive me” which clearly proves original sin is present at conception which necessitates a spiritual soul. The answer escapes me and causes me to question if they were they really so brilliant. If they were brilliant, then why the primitive thinking on ensoulment?