Because their cultural and racial memories determined it. You use it, and you can lose it, as did the Mythic Dawn who killed their mothers and joined Mankar.
ok, but when I'm saying is that if Lamarckism did indeed apply, every creatures in TES would be covered in scars and be born with the inborn ability to have rank 100 heavy armor skill. That obviusly doesn't happen, so Lamarckism doesn't really apply. If anything, the evolution is directed by organized/intentional mutation (ie Dunmer)