To see why, take a look at the Watts Bar 2 nuclear power plant. Watts Bar 2 started supplying power in June of 2016, becoming the first nuclear power plant to be built in the U.S. in two decades. Overall, Watts Bar 2 was the result of 37 years of construction. For most of that period the reactor simply languished in a sort of economic limbo. Construction was halted in 1985 due to low energy prices and only resumed in 2007.
At that point, Watts Bar 2 faced another decade of construction delays and cost overruns. The reactor was initially scheduled to be completed in 2013; delays pushed back the start date to 2015 and again to 2016. The initial cost was estimated to be $2.5 billion; the final cost, after a series of unanticipated hurdles, was $4.7 billion.
The truth is that with its delays, cost overruns, and other woes, Watts Bar 2 is a typical nuclear reactor project in the U.S., not an atypical one. Reactors are gigantic beasts, and sustaining a nuclear reaction while drawing power from it requires an absurd level of engineering. Reactors are expensive, bulky, and complicated, to say nothing of the waste products they produce or the fear of a catastrophe like Chernobyl or Fukushima.