Because the public can see their crappy audits and complain enough that some congressman sees an opportunity to clean that crap up so he can put "saved billions of dollars by rooting out waste" in his re-election TV ad and peddle to a special interest group (the people who want to minimize government waste).

At least that's how the theory goes. You need people to actually care instead of just shrugging and saying "well that's government, nothing you can do about it" like they do for my state government (though I have lived in other states where the general populace did not seem to take government waste for granted like that).