For the same reasons all the real world cultures (mostly tribals, go figue. Might have something to do with other government forms requires a higher level or organisation and infrastructure...) did not adapt the technologies of their neighbours...Fair enough. But, paleothic or neolithic humans had limited access to newer technologies. After all, those cave paintings were the cutting edge of art of the time.
But, you've still failed to explain how a group of people, who trade with the greatest repository of knowledge in the land, and who live less than a day from a major city (by boat), lack tools or technologies. Sure, I can understand some remote island culture who have never made contact with other people might lack technologies, or a group of people surrounded by a honking big impassable desert might not have access. But, can you explain why a group of people who have regular contact, to the point where when they stop delivering fish, it's actually something of a problem for Candlekeep, and have traded for years, lack advancements.
Bonus points if you can explain why it is ONLY tribal peoples in the game that are "primitive".
Pick one you like.
But we are talking in circles now.