Batteries go bad and leak and eat away their surroundings.
Capacitors degrade and can leak and eat away at their surroundings.
IC/Chips can have acceptable manufacturing imperfections that can get exasperated over time.
All the above cause issues for the retro-computing scene and apply just the same to our train electronics.
I'm sure there are others as well - and this is discussing JUST shelf life - not operational life.