Brass of the socket relaxed with time and thermal cycling, and this reduced contact pressure meant the current flow was going through a smaller area, which then heated up and this then exasperated the problem as it reduced the tension more, and the heat oxidised the copper to reduce contact area.
That is why they normally use a spring steel backing spring on those terminals, and use a grease to keep them clean and corrosion free. For high voltage and reliable ones they use an oil filled housing, as that means there is no corrosion even after decades of sea exposure, with only a maintenance cycle every 5 years to check and repair outer housing corrosion. The inner side looks like new.
House fuses are rarely changed so the dry contacts do not get a wipe cycle to clean them and verify the contact tension is sufficient ( means you need to use a lot of force to pull out both sides, and they do not just fall out), and the linesmen and installers rarely use the grease on them, even though it is a recommendation of the manufacturer.