Maybe I'm ignorant but my understanding was it's not really a social construct for us, it's often rooted in the understanding of genetics that makes inbreeding a bad idea.
Mostly myth. The first million years nobody cared. Inbreeding over time is a bad idea, if the gene pool is too small. But line breeding(?) like done with dogs (and cattle and pigs and ...) is not a death sentence, nor even a very bad idea unless taken to extremes.