Well, one answer may simply be that militaries are class-and-tradition-laden bureaucracies that are hostile to change. Certainly this has been the experience of the US when attempting to build a western-style NCO corps into the militaries of allied developing nations.

Another answer might be that it's already possible in principle: somebody has probably already mentioned this, but one can go from a NCO role into an officer role; the traditional term for this is "mustang officer".

Another answer might be division of labor: traditionally, western military officers are required to gain a lot of breadth in their careers, both through academic education and through assignment to different types of field commands, logistical positions, and political positions. This poses a (common) problem for officers who have a personality type that wants depth in one type of military occupation; that role is more traditionally a senior NCO role. It may be, however, that this type of division of labor produces the most effective fighting force; I don't know.