As an electrician I worked on gov't projects with private contractors. In every situation there were cost overuns due to constant and often unwarranted changes. It seemed like every gov't official wanted his say on the project...Some might call this corruption...
It might be corruption in some or many cases ... but the main reason that (unlike the French) we do not and never have had a standardized nuclear power plant design is because "Everyone Wanted His Say", whether it meant more money for them or not. Every one of them is a custom-built one-off, as if we are starting from scratch.
The reason is that the utility operators want everything just like THEY have always done it, which is different from everyone else. When we DID try to implement standardized French designs for American power utilities, the operator wouldn't have it. If something was red, it had to be blue - if it was a pushbutton, it had to be a lever - if it was 250 volts it had to be 249 volts, or they wouldn't sign off on it.
So we never got to amortize the work that had already been done, the NRC had to approve new design changes, and the costs skyrocketed. Maybe someone was making some money on the side, but most of it was "I'm In Charge Here, and It's Going To Be Done MY Way!" even if that "way" was demonstrably inferior. Nobody was in charge to tell them "Here's the new power plant - the rest of the world uses it, get used to it."
It's human nature. We even see it in the infamous "Oil Threads" on our bike forums. Nobody's got any good reason to use something besides the manufacturers' suggestions - they just want to be different, to be part of an Inner Clique that knows the Secret Sauce, and not look like they're being anybody's "boy", especially not to the guy who wrote the manual