Because the money for modernizing, let alone replacing those plants is a lot better invested in renewables. Battery Storage is very close to store baseline energy, not in Lithium Batteries though and the projected cost for Redox-Flow Batteries is going to be falling like crazy. It already is on the same level as Lithium, and while it is less energy dense, it's safer, easier to operate and especially longer to operate. The Dalian VFB in Liaoning, Dalian, China is one of the first "bigger" stations to come online, but there are already a lot of plans. I also have one in my house.

Most plants are to old to modernize and building new ones is expensive, takes decates and dosn't solve the dependency on uran and the nuclear waste problem. Renewables were always the longtearm goal and gas for shortages. Nuclear cant be easy switched on or off, so they aren't a good solution to help with energy lows.