While the tiered bonuses are potentially questionable the +1/2/3 weapons and armor also represent the generically magical equipment in myth, folktales, and fiction. A lot of magical weapons in stories just have the quality of "being really good", so how do we represent that mechanically? A flat bonus to hit and damage makes not-Excalibur clearly superior to a mundane blade of even the finest craftsmanship, without loading it with extra powers and fictional implications that would come along with making it, say, a Flametongue.