The lack of accountability happens because the government is not allowed to build up the in-house expertise they'd need to manage projects like this, because of this bizarre American idea that government=bad private=good. So the government agencies end up having to outsource absolutely everything - and not only is that expensive, it's also doomed to failure, because understanding and accountability by its nature can't be outsourced. And that's only compounded by a lack of trust, low-bid rules and the like.

If you look at the entities that have actually managed to get stuff like this done (which is something most developed countries around the world manage), it's not private industries, it's governments - but governments that were enabled and trusted to build-up effective in-house teams for doing large projects.

But why are other countries able to build successfully, despite the same chain of accountability?

The problem is the government doesn't know what to ask for.