All three of the programs you name are untouchable because they are funnelling money directly to powerful political stakeholders. Being real blunt here, poor people don't make political contributions and they hardly even vote. For the most part, they are easy political marks and cuts to general-population social programs happen regularly.

There are notable exceptions, like senior citizens - which is why we haven't done anything about the unfunded expediture on the Medicare prescription coverage.

While I won't discount your claim about hidden agendas behind much bureaucracy, I don't think you can really apply this to two of my three examples. For ethanol subsidies, sure. But it'll be much more difficult to make the same argument about the War on Drugs or the TSA. In these two cases, how is money being funneled to powerful political stakeholders?

Police departments and corrections officers of all types. The TSA is a giant jobs program for workers no one else will hire, and the body scanners are produced by a company owned by the Secretary of Homeland Security that mandated their purchase.

Good answer. Do you think it's always the case, then, that any governmental program automatically creates its own constituency?

It should be obvious that government agencies lack the incentive (generally profit and the competitiveness necessary to maintain it) to meaningfully address and/or eliminate problems. Nor do they experience any significant downside to failure. They aren't legally accountable to shareholders and it certainly isn't their life savings that they stand to lose. Worse-case scenario, they're reassigned.

Government agencies aren't expected to innovate, they're legally required to carry out the will of Congress and do absolutely nothing else but the will of Congress.

Most of the "failures" you see are the result of a dysfunctional political system. For example, the Post Office was looking to branch out into services like bill payments, identity verification, banking, etc but Congress forbid them from doing it. Not to mention the crazy requirements Congress set for pre-funding their retirees. No private company has to do that - most of them are pushing their funding obligations off to the future. Underfunding retirement isn't a good idea by any means, but the playing field is most definitely not level here, both in terms of responsibilities or ability to pivot.

It's wildly unfair to blast government agencies for not innovating. They don't have a CEO or a board, they have Congress calling the shots. Of course they aren't agile. And even worse, half of Congress thinks the entire concept of government services is illegitimate and is actively engaged in sabotaging and defunding those services. There's not really a parallel in the private world where a CEO is destroying a company because he thinks it shouldn't exist.