There is a practical level where the ruleset doesn't necessarily have to cover every single contingency or edge case, particularly in don't-have-tos ('you want your elf to be raised amongst dwarves and as such never learned swords and bows? Simple solution: don't use swords and bows'). Particularly since in the current version of the game, weapon proficiencies in isolation are a minimal part of combat prowess (if you really want to be good at fighting, you grab a martial class which will have all the proficiencies anyways; if you are an elven wizard, getting to use a rapier or longsword for 1d8+stat damage instead of the 1d4 a dagger would bring will not change whether you are a competent melee combatant). That said, since the game has started to split out character creation in terms of background as well as race and class, there's no particular reason why they couldn't (in the next edition, for example) split out species-that-raised-you from biological traits.