Why Inbreeding Doesn’t Cause Defects

Humans suffer when we inbreed because of the efforts taken to keep us
alive and allow us to continue to be parents, avoiding the natural
selection process, in particular modern medicine.

Inbreeding often happens in the wild but, as long as the breeding pair
does not share the same deleterious genes as their partner, their
offspring can be expected to thrive. The greater the chance of sexual
partners sharing deleterious genes,the more likely that offspring will
be ill.

This problem – an increasing deleterious gene load- is happening
rapidly to humanity with the help of medicine, and some experts predict
that unless science finds some way to avert it, humanity may face
extinction from this threat alone (never mind the other threats!):
harmful concentration of deleterious genes. Good argument for eugenics.

Also consider that if the offspring of an inbred couple is healthy they are likely to be clear of the deleterious genes carried by their parents.  Inbreeding can be like a clearing out process to quickly bring the defects to the surface, which Nature would take care of  normally.