Opening up DNG files

On my Mac I can export a jpeg or Tiff file to my desktop, put the mouse over the file, right click and select “Open with” and I get a drop down menu with at least 20 selections including all the nik collection. But when I do the same on a DNG file only 4 selections are there and the nik selection is not listed.
Can someone tell me why?

First it’s most likely because DNG, unlike Tiff and jpeg, is not a universal standard file format. It is a proprietary format created by Adobe. Adobe wants DNG to become a universal standard raw format, and perhaps some day it will be, but it isn’t one yet. Second, unlike the other formats you mentioned, DNG is effectively a non destructive raw file format and many software packages simply are incapable of being able to edit, or even view, raw files. There may be additional reasons which I’m sure others will mention.