Because there's no routable path there, and if there's no indication of such a path being there, it should not be mapped. There's cases of informal paths @roux2scour@jlai.lu's picture describes, and may be tagged with "informal=yes". Other than that, I'm not aware of any routing services, which would behave like you'd want them to. The path of the starting point also seems raised (as being on a hill), compared to the endpoint; so I'm not sure how desirable this particular case would be, even if it were to be supported.