I have an impression that they're are saving oil/profits for some possible "black day" in the future. And, the "profits" which are shared among all citizens come from high taxes. I might be [partially] wrong though.
According to Wikipedia, there are discussions about how much of the oil revenues should be saved; opponents of using more of them directly say that it would just lead to inflation.
https://en.wikipedia.org/wiki/The_Government_Pension_Fund_of...