The elderly arguably have grand bargains protecting them. Young people pay into Social Security to meet the promised payments to current old people, in the expectation that future young people will do the same for them, and so on.

Like wills. Why should we execute wills that leave millions to pet dogs or something? The dead person is dead, beyond any caring. Just take their bequest and do something useful with it! But of course, if we did that then people writing wills no longer trust will executors and will disperse their assets in life or just waste them. A grand Newcomb-like bargain.

No such bargains protect children. They haven't provided anything and won't be in a position to for a long time after a huge investment. (What's the estimate of the net society-wide cost to produce a finished high-schooler? A few million? Highly non-trivial, let's say.)

People on average increase in societal value from conception to childhood, and then it gets more complicated from there depending on how they turn out. And yes, typically their value declines as they become elderly.

But, as in your example with your adopted friend, even a baby that starts out unwanted, if society invests a bit in its welfare, will soon become part of the social fabric and so on and thereby become valued.

Certainly there are some people who literally nobody likes, but even then, there's still reason B.

As it happens, my best friend was adopted as well. But I hardly think the limiting factor in the number or quality of my friends is society's production of babies.