Why Do Translators Include Passages They Know Are Not Original?

Based on what I have said about the textual variant of 666 and 616 in the book of Revelation, several readers have asked a distantly related question.  Here is how one of them phrased it:

 

QUESTION:

If the biblical scholars know with certainty that Mark 16:9-20 and John 7.53-8.11 were added by later scribes, why are they still in the modern bibles, that is, why are they not *completely* removed? I know these verses were removed in the RSV but added back in the NRSV.

 

RESPONSE:

This is a gre...