What kind of self-regulating system of taxation are you talking about? Lol. Politicians couldn't keep their hands out of the budget. What they do and politics? Maybe you believe that they go into politics to make the country better? They will never give control over taxes and nothing to even speak on this subject.
You are absolutely right, but they can't declare their true intentions in public. Instead they need to declare idealistic doctrines, and then, in democratic societies - to implement something that will show their devotion to the declared course. Quiet so they surrender some power - by little steps.
On of my most beloved Frederick Douglass quotes: "Power concedes nothing without a demand. It never did and it never will"
So we need that demand, and we need it to be loud and clear to have chance to make authorities make things we want them to do.