BECAUSE NEITHER OF THEM HAVE A HEART ! THE OTHER TWO HAVE HEARTS I.E Sora and Kairi. Xemnas wanted a heart and Xehanort wanted a body with a heart.:cursing:Why didn't Xehanort's Heartless and Xemnas combine. They were both in existance simultaneously and they were both in Hollow Bastion at the same time. They could have just combined and made a complete being since for one that would have made their goals that much easier to be accomplished since, Xemnas wanted to be whole. Xehanort's Heartless needed a body to inhabit thus, him possesing Riku. They could have easily just came together like Roxas and Sora and Nimine and Kairi did. Especially since they both wanted Kingdom Hearts and they both obtained Kingdom Hearts but, in different way so they basically had the same goals just acted upon in different ways.