Abortion is legal because mothers are treated as the Owners of baby daughters and sons, who in turn are treated as Property, to be disposed of at their mother’s whim.
That state of affairs is, in turn, the direct result of society disenfranchising fathers.
You are overthinking a system that in the crude psychological sense grew out of people thinking with their genitals. Whatever there is that helps there be more sex without concern for consequences is deemed right and just.
It isn’t that they had to deal with abstract thoughts about personhood or ownership … most just didn’t care about such nuance in the least, so they didn’t bother with them.
So really “you” didn’t overthink anything, but are only attributing thinking to those who often weren’t thinking at all.
As for the underlying cause to the crude psychology, I would suggest that it has to do with spiritual depravity. The same depravity resulted in things like demand no fault divorce and the marginalization of fathers, not the other way around.
Abortion exists within a spectrum that includes easy divorce, approval for and championing homosexuality etc that are all outward evidence of an underlying condition. Abortion is a “worse” among these because it is about as unloving as unloving can get, a crass betrayal of all natural affections.