I generally agree with the sentiment, but there's no shortage of adoptive parents for infants. The desperate need is for older children, which is a very different kind of experience.

Imagine that there is a neurotype that loves the idea of adopting (I have actually known people like this). So they adopt children in lieu of having quite so many of their own. Or they have none of their own at all. Nature, our harsh mistress, will weed people like this out of the gene pool very quickly.

Now imagine that such a neurotype is an environmentally-mediated part of the phenotype, not a strictly hereditary trait.

Maybe it's called "Amazing Grace," or maybe it's called a principled commitment, or maybe it's called love. And experiencing it from others (maybe by being adopted by them) inspires it in you.

Then, our harsh mistress would in fact tend to favor its expression and grow its prevalence among the population.

Doubtful. Cultural practices are subject to the same selection dynamics that genes are. A cultural group that doesn't create its own children will eventually die out unless they have some other group producing new members for them at a very high and stable rate. See shakers for instance. Dead. Gone.

Contrast that with homosexuals, who haven't until recently been having children en masse. But where there is a certain percentage of the population born with a tendency to be gay. So they can always count on having new members.

Unlike being gay, people aren't born with "Amazing Grace" so ... it's a bad strategy that will be weeded out over time. Just like with the Shakers.

This isn't to say that some highly fecund cultural group, like Mormons, can't get away with adopting children here and there. But if they start going too far with it their numbers will start to dwindle.

There are not all that many babies.

Older legally free kids do gain awful lot by being adopted. It is generally also very hard to adopt older kids. Not everyone has what it takes. And when people fail, the failure is euphemism "they got abusive unable to deal with very real issues doing awful lot of harm".