Perhaps it'd make more sense if the horse dudes were iron-users. But even so iron/steel weapons would be relatively rare, with wooden-hafted axes or picks (with metal heads) far more common than swords, and the average arrow still possessing a stone or bone head, and only the most elite of military forces or the richest merchants able to afford steel to spend on arrowheads. Armor would be largely leather, with sparse bits of chain or perhaps lamellar covering the vitals in the front, but not much else.
Or hell, maybe they aren't raiders, aren't really great warriors, just have good horses and enough steppe to run away from any pursuers for a very long time.