Why seperate messages for each controller analog axis?

For a given joystick/controller analog stick, why generate a separate messages for each axis? Mouse move has the x and y in the same message. The only thing I can think of is some old joysticks could be twisted for a z axis. But isn’t the controller interface designed to unify controllers similar to the xbox controller? I don’t think that would be the case for the controller interface and the x and y message can be unified.