Most of the villains are given at least a modicum of characterization, though. The members of XIII we meet in CoM are all well-defined as characters with their own agendas and motivations, and even though KH2 was more reliant on identifiable tropes in attempting to differentiate the remaining members from one another, main players like Xigbar, Saix and of course Xemnas pointedly stood out from the crowd. These characters have defined arcs which continue to inspire interest and discussion.
Young Xehanort, by contrast, is so lacking in innate motivation that the writers literally had to invent a reason that his presence would have no impact in order to keep him from accidentally doing anything meaningful.