So for those of you wondering about the dagger, you have to think about the setting for the books/show.
Westeros doesn't have communication tech. They send short messages by raven and even that is unreliable at times.
It is before the printing press so copies of books are written by scribes and there isn't a good number of them out there. Knowledge is not common.
In fact, unless you were raised by a noble family with money, you are illiterate. This is why you see Davos Seaworth learning to read.
This is the same reason you hope to gain favor with an noble house so your child can become a squire. Again, I point to Davos, a low born, who saved Stanis with his onions to become the Onion Knight, so the son of a knight can become a squire. That's why Davos' boys are literate and he is still not.
Now consider all that, PLUS it is a world where magic, grumpkins, dragons and wights have become extinct. Almost nobody remembers the true purpose of the wall, it was built to keep out the Wights and Others. In the time of the books, this has been long forgotten and people think the Wall is to keep out the Wildlings. This is why the Night's Watch is really only running on tradition and explains why only 3 of 19 Castles are manned, why they have few people and no resources.
So if they can forget about why The Wall was built, they can surely have forgotten about Dragonglass and it's usage.
In the book, Sam spends time in the library away from his brother and reads a lot. He was tasked with finding out about the others, and he had to dig through the archives to find the info. This is how we were introduced to the Dragonglass. It is
that forgotten in a world where information and knowledge is already scarce.
Any questions boys and girls?