Living things that pursue completeness prefer crossbreeding. Even primordial cells knew this. Instead of blocking germs that threatened to infiltrate them, the cells sought co-existence and co-prosperity with them. Today, the mitochondria in our somatic cells provides evidence of this phenomena. Even if we humans do not consciously choose crossbreeding, our instinct for it is certainly in our DNA.
The same principle applies to education, where academic inbreeding decreases competitiveness for both students and schools. Educational institutions that succumb to this problem fail to keep up with current trends, which is why Ivy League universities in the United States often hire from outside their ranks.
While academic in-breeding goes well beyond simply being at the same school, this is a significant problem.
These professors primarily graduate from a SKY university and then do their graduate studies abroad. This certainly lessens the damage of in-breeding. However, this also ignores the cadres of qualified, talented applicants who happened to either bloom later or thrive in other environments. It is a terribly short-sighted.