Why are de novo genetic mutations usually NOT inherited?
A germ line mutation will be passed to the offspring only if


the individual reproduces
the individual reproduces after the mutation has occurred in the germline. Indeed if the mutation appears in the testes 5 years after the individual has reproduced then the offspring will never carry this mutation. Note that female mammals make a big storage of ovules early in life while mammal males do not. As a consequence the number of new mutations transmitted to offspring is age-dependent in males but age-independent in females. See this post for more info
the gamete that is transmitted is descendent from the individual cell who had the mutation. There is of course many lineages of gametes and their pattern ancestry will affect what fraction of gametes carry a given mutation.
the right of the two chromosome is being transmitted (last point applies only if the mutation occurred before the meiosis). Of course, there is always segregation who decreases the probability of a new mutation to be transmitted by half.


Of course, there are also all the mutations occurring outside the germ lineages (somatic lineages) which won't be transmitted but you specifically asked for germ line mutations.