Because it would be kind of neat to see these little touches to show that this isn't your standard generic medieval fantasy setting.
So, full disclosure. I'm not the writer or project manager for this product. My company is two people; my partner Jason Sheets is the main writer; in this project I contribute the non-written content and the Lore 100 System. And the Lore Sheets. Anyway, I'll talk over your suggestion with Jason.
Personally, I don't care about costs. But I guess this is an important thing for D&D players? (Actual question... I don't know). A lot of GMs (like Jason) are able to quickly spit out a cost for players because over the years they have done it so many times that they know how much, say, a long sword costs.
When I was making the costs lists for Lore 100, I was basing that on D&D so that we don't need to change a lot of text. But as I was looking at the D&D equipment list I thought, "wow, this makes no sense whatsoever." I'm not talking about the use of gold; I'm talking about the relative value of items. A spear is 1gp while a dagger is 2 gp, for example. Long bows costing more than long swords.
To make the change, I believe I would need to change the relative pricing of weapons in D&D. Which, maybe, is a no-no with some fans. I would then need to change descriptions of gold to silver, and silver to copper. Not a big deal but does take some time. Then I would build out a price list based on medieval pricing. Two problems with this: 1) the game is a fantasy game which nominally takes place in the 5th century, a time where we know very little about, and 2) does this potentially invalidate any other D&D mechanic which is normally used in the "progression curve" of players. I myself don't care about either of these problems because I don't feel a need for historical accuracy and I care about story progression, not PC power level progression. But many players, customers, and backers may feel differently. What do you think about this?