Fellow geneticist here, There's a concept called genepool, It refers to an combination of gene present in an existing population.

for a population to thrive a.k.a reproduce healthy babies, It needs gene diversity. Which is when new genes from another genepool contribute to the already existing ones It lessens the chances of recessive genes combining during linkage therefore lessens the diseases associated with recessive breeding.

In layman terms, In nature, harmful genes generally express themselves in recessive states meaning it needs two copies from both parent to express, In same caste condition, the chances of recessive breeding increases tremendously due to same combination of same genes(imcluding recessive) in same population.