They go over budget because regulatory burdens are very high, not because of any fundamental unknowns in deploying the technology. A motivated national government could reduce the cost substantially.
Regulations of course have a purpose but they are often (a) out of date with respect to current technology (b) not well well designed to accomplish their intended purpose in the first place or ignored 2nd and 3rd order effects and (c) perverted to extract rents for incumbents at the expense of new market entrants. Certainly (a) is true in most places with respect to nuclear technology.
