Because an NPC is always there, in every town.If it was just as easy to buy it off the broker as it is off a vendor npc then what would be the difference for the end user?
I am a strong believer that such roles should be left for players to fill in a game like this.
High in demand items are exactly what the economy needs. Why waste that opportunity by having an npc fill it?
I would hate to have to travel across the world to Tind or wherever just to buy bandages or arrows. Even if SV greatly expands bank storage and broker capacity.