Supply is high because this stage of Smithing is rather barren in terms of cost:benefit synths. These arrowheads are conisdered the relatively-cheapest way to skill-up.
Demand is not terribly high because one purchase is 99 synths for top-end Woodworkers that also have 50+ Bonecraft. Kabura Arrows are a great way to get Woodworking from mid-80s to 91, but the supply is roughly 6-times the demand.