Showerthought: Relics generate gold because they represent churches getting increased tithes.

Relics were famous religous artifcats purported to be the body parts of saints or famous artifacts. Churches would often use these as a way to bring about pilgrims and ask for more donations from wealthy travelers.

Relics typically start out small and only amass major gold when you store more than once. That monastery, for obvious reasons, tends to be sheltered deep in your main city.

So they generate the most gold as pilgrims flock to your capital, paying tithes and entry fees to your most famous monastery. They're actually a form of trade income.