The reason for this is, of course, because children's stories are often about families, don't need bestiaries, and don't need to make any sort of particular sense - that's not the point. But nonetheless, I find the implied settings in which these semi-unique creatures live fascinating. Worlds in which a single family or a small group of similar beings can exist on its own, living on its own terms, without being part of a bigger whole.
In fantasy for grown-ups, the semi-unique monster takes on a slightly disturbing tenor that isn't present in children's stories. Isn't there something terrifying and horrible about the idea of being part of a group of half-a-dozen creatures who are all there is of your species? I don't mean because of the threat of extinction; I mean because however hard you search in life for a sympatico, a soul mate, somebody who truly understands you, well, this is it, the entire pool you have to draw from.
There's also something compelling, though I can't quite put my finger about what it is, in the idea of people in a D&D world being able to refer to an entire creature type, found nowhere else in the world except in their little local 6-mile hex, as a collective noun. "Watch out if you are travelling through the Old Forest tonight. That's when the pontipines come out." Is it just because it harks back to the kind of thing I might have read in the tales of my childhood? Very probably, but I like it, all the same.