It costs extra because CORRUPTION. They're skimming off the top at every level, and that's why they can't let anyone investigate.
Just simply incompetent (at best) people milking the projects for insane amounts of money while adding no value. Actually in many cases adding negative value.
Basically low-level corruption like scheduling an extra couple construction laborers to a project that only needs half of what you assigned.
It's the same problem with the means-tested entitlement programs in the US. Sure outright fraud (e.g. welfare queen) is not worth talking about it's so rare. But the day to day low-level fraud that is endemic and pervasive to the system? Never counted in any studies.
The general incompetence and fraud has slowly (the older I get) become the norm. It's rare you come across competently done projects these days that are not swamped by fraud and waste.
I guess my point is that it's so common it's simply become normalized these days, and people no longer even see it as fraud/corruption/sloth - it's simply the way things are. It's not just construction - it's practically everything. I guess it's pretty similar to cost disease.