That's because yesterday, my fiance, Ted, opened up our 1933 Oxford Universal Dictionary (one of the three I had accused of neglecting the word), pointed to a page and said, "Didn't you see this?"
It was right under my nose.
Hallow: usu. in pl. hallows. 1. A holy personage, a saint. ... 2. In pl., the
shrines or relics of saints, the gods of the heathen or their shrines.
And lo and behold, with one reference to "relics" we get a really solid clue about how the word evolved from "saints" to "spooky stuff."
On a similar note, did you know that the word "gullible" is not in the dictionary? Better yet, would you believe that I fell for that gag not once but twice?