"First off the budget doesn't need much flexibility at all."
Were you not around when New Orleans was flooded?  Are you unaware that Alaska is prime earthquake country?  Or how about another federal program that gets cut (for instance highway funds) because of a new administration's policy changes?  Or . . .  I just don't understand how you come to this conclusion.

"Secondly I do not understand why the assumption is that everybody is so bloody selfish. Why do we assume that ordinary people become corrupt once they get in office and can no longer effectively budget?"
That's a better message as I can understand where we disagree.  I don't think the problem is being "bloody selfish" but, as we saw in the last budget (for a budget shortfall) or the 2008 budget (for the last example of a budget windfall) intractable disagreement of how best to create revenue and spend/save it.  This isn't to say that legislators aren't selfish and desire re-election...