The reason why elite MBAs make sure to ensure the success of each other is to make sure they're also given the leg up when looking for opportunities.
If a Oxbridge MBA judges a compatriot solely on merits and doesn't extend an opportunity then it cheapens their own value.
Perpetuating the privilege is the point. It's the only reason why I would do an MBA - for the blatant, proud and culturally accepted nepotistic advantage.