It is not hard due to lack of knowledge, it is hard due to politics, and the fact that they require trained / skilled builders and operators.
So it’s a cold start problem. As we aren’t making many, we don’t have much trained staff.
Haven’t we been building reactors for decades though? Are all those guys dead along with the COBOL programmers?
We have documentation, but you.need to have construction crews working to keep the specific routines and needs sharp.
Sure they have people “ready” to work on such projects but it takes significant time to tool and train up, even for the “ready” folks because they don’t do these jobs often.
Are they significantly different from normal specialized plumbing/electrical/heavy construction work? Weird, complex things get built all the time these days and I’m curious how much different pouring concrete for a reactor is compared to, say, a bridge.