The United Nations fulfills a number of roles but perhaps its foremost function is to act as an arbiter in international disputes. To do that effectively it needs to reflect the opinions of the international community and deal in realpolitik. As things stand that would make it impossible for the organisation to take what would be seen as a partisan stance.  Recognising the existence of a state which could at best be described as aspirational, and at worst as a fantasy, would put an intolerable burden on the UN’s ability to act as an impartial agent in negotiations.