With Hammer, guardian has three attack skills, a condition removal, and a protect. Warrior has five attack skills.
With Mace (main hand), guardian has two attack skills (one that heals), and a protect. Warrior has two attacks and one protect (which also causes damage). I'd be surprised if the healing guardian skills does as much damage as a warrior attack.
I'm not going to compare every skill, but I stand by my guess. Plus, we know guardians inherently have less HP than warriors, so they HAVE to be more defensive to survive.