Why are some airfields not on any sectional chart nor listed in the directory?
There are lots of airfields not listed on the charts or in the directory. For what its worth there is nothing that requires private airstrips to be listed. As mentioned in the comments sometimes its not worth the hassle. As you mention in the original question it is a nice gesture to list them for other pilots in an emergency. That is covered nicely in this question. They may not have listed it because they don't want people doing touch and go's there, or landing there and knocking the door of their house or they did not want to up-keep it etc. etc.