You might want to ask _why_ it is "very common and repeating". There is
strong circumstantial evidence that it is "very common and repeating" for
the same reason that most female honeybees are sterile workers; it is
to the long-term advantage of the genome to express a phenotype that is
not optimized for reproductive success but that enhances the reproductive
success of its close relatives. There's no "just so" about this sort of
phenotypic oddity; it's an evolutionary adaptation that is or was useful
to us.
A good starting point if you want to read about this is "The Extended
Phenotype" by Richard Dawkins. If nothing else, it may make you question
the many facile over-simplifications of Darwinian theory that we are
constantly bombarded with because they support various ideological
positions.