Life has always been a "crapshoot" at any stage of development. It's only relatively recently that we've cut down on infant and child mortality, and pushed up the average life expectancy.
Throughout most of human history, infant mortality was horrifically high and half of all humans didn't make it past childhood. Were children only worth half as much then?
We're talking a huge portion
not even being known in the first place, here,
on top of mutual exclusivity in survival of one,
very rarely two, and any more being statistically insignificant, among
dozens.
You seem not to grasp the sheer scale of just how
impossible it is for the
extreme majority to ever take on any recognizable material realities of personhood. If for no other reason than logistics of wombs to house them.
It isn't just a minority making it to five years old due to now-solved risks. It's a minority being
capable of making it to birth, whether the limitations of the mother or the zygote being quite literally unfit for life.
And, of course, people
did place dramatically less value on children back then. Christianity had to make a point of infanticide being wrong for a
reason. Dangerous child labor was normal for
decades of the Industrial Revolution.
These bounding factors are so severe that it is literally more common for one zygote to end up two or more people than for two zygotes to come to term from the same pregnancy.
Edit: Okay, actually checking the data, it turns out it's a
drastic crapshoot based on a load of different factors, but a very important note is the estimate that around 1/8th of pregnancies
start as multi-fetus but one outright
disintegrates without a trace. Which is
after all the fertilized eggs that never implant.
Fundamentally, the point remains that "Life Begins At Conception" means the
exceedingly large majority of "humans"
literally cannot be born by any current means. Which means any valuation of life that actually puts a
value on it instead of ivory-tower idealism calling it immeasurable would
plummet dramatically. As was the case with historic infant mortality rates.