Because they are aquired traits.

The traits which are earned by us during our lifetime are known as aquired traits and these traits don't belong to the dna or any of the characteristics and more over its only to the physical part and this does not create any chemical changes in our body so ,traits are passed to the progeny only if the traits are chemically connected to our body and  if they are inherited from our parents they will be passed if not they will not be passed  . To say in simple words traits which are present in DNA will be passed on and traits which are acquired by us and there is no linkage with the metabolism and DNA
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