Jamison Foser has a question:
The real answer, of course, is: "Because they'd tell us that spending cuts will hurt the economy and be bad for jobs. And that's not something we have any interest in hearing."
But perhaps you can think of something better. What excuse should Republicans use? It's a waste of precious CBO resources? The CBO is hiding the truth from us by refusing to use time-tested supply-side models? Doug Elmendorf once went to dinner with somebody who was friends with someone else who invested in Solyndra? There's gotta be something.