You might want to look up flammanville. They built a new reactor there and that also took 20 years or so and was way over budget.

We've built a lot of nuclear in the last century and then largely stopped. A lot of the know how is gone which is what we're paying for now.

Also, in France, all those reactors were largely the same leading to economies of scale when building them. Everything we build today is essentially a one of so you don't get to spread that cost over multiple.