A perenially tempting idea in financial policy is hypothecation (or "earmarking"): dedicate the money raised from tax A to pay specifically for thing B, as distinct from putting all the revenue into a single account and using ongoing political processes to decide where it's spent. In the naïve household accounts model of national finances, this makes sense: the money I save from giving up smoking goes into the holiday fund. Why, in general, doesn't it get done at the national scale?