[zaarn]

Bigger and more crudely build components probably play a large part. A Microprocessor from the 70s and 80s is made on much larger nodes than today's chips, traces and soldering points are bigger, PCB's thicker, almost all components sans resistors and wires were either bigger or build differently.

They have more material to play with while they decay compared to modern devices.

[ido]

I'll bet good money that an IBM PC or PC XT that has been properly stored will boot up and be more or less as usable today as it was the day it was put to storage (especially the floppy-only models without an hdd).

[zaarn]

Possibly, though I would still check capacitors and any voltage regulation (transformer and linear) to make sure it won't blow up suddenly because some wire coating rotted through.

[ido]

I guess another candidate are any batteries leaking, but IIRC the PC & XT didn't have battery backed clocks & therefor no battery on the motherboard.