Here's why:
* They are attached to an XYZ Monster: XYZ Materials are attached to the XYZ Monster they were used to summon, not placed on the field independently.
* They have no independent effect: They don't have their own effects or abilities while attached to the XYZ Monster. Their sole purpose is to provide the materials for the XYZ Summon.
* They are not treated as cards on the field: For the purpose of game effects and interactions, they are considered part of the XYZ Monster, not separate entities on the field.
So, while they are a crucial part of the summoning process, XYZ Materials are not considered to be "on the field" in the traditional sense.