Because many of them were built before TMI and Chernobyl, in the 70's using designs from the 60's. It's similar to how maintenance on a car from the 70's probably isn't worth it.

Nuclear isn't working very well in France either. The reactor in Flamanville had an initial projected cost of 3.5 billion euros which has now ballooned to 12.4 billion euros and the project is over a decade late. https://www.reuters.com/article/us-france-nuclear-idUSKBN24A... The French are also preparing to transition away from nuclear as more plants are being decommissioned.