People do often act stupid, but you are seeing it from what I consider to be an incomplete perspective. Nuclear could be financially unwise overall, but someone would still get a payday. That 17B over budget wasn't burned and unmade, it went into the pockets of the people organizing and building the power plant.

All this to say, the huge majority of the people involved in making the power plant a reality weren't motivated by the efficiency of the power production on a cost basis. Most of them were probably making more money while it was still being subsidized, planned, and built. And while I think subsidies are generally useful and good, they can be a vector of financial abuse when it comes to unprofitable industries.

Lastly "lifetime cost" is a bit of a useless metric when the majority of that lifetime comes too late. No point to a power source that will cleanly produce power after it has meaningfully contributed to pushing us over the edge and past the breaking point for a climate that can support agriculture as we know it. There isn't enough time or margin for error in emissions left available to build all the nuclear plants needed to meet energy demands.