They have devices in them that break over time as all devices do, and those devices have parts and designs that were contemporary before the people working on them today were alive likely with architectural and design decisions that were operationally required back when things were being drafted that no longer make sense to do today. Likely the nuclear materials will be reused, but that’s me thinking with my brainbox and not actually a thing I know.
For an example of what happens when we continue to rely on tech that really deserves to be updated and/or replaced, see the United States banking sector as compared to basically everywhere else.