Software can decay, hardware can decay, both can exacerbate each other. The registry gets filled up, the filesystem gets fragmented, the software accumulates in memory, upgrades of the same software accumulate hacks and poor design leads to worse fixes to problems, and slowly buggier versions are shipped. The hard drive slowly fails, the SSD loses viable blocks, the fan and heatsink gets clogged with dust and less efficiently cools the system, the battery loses capacity which causes the system to throttle itself, the power supply components slowly degrade causing voltage inconsistency, tin whiskers develop.
The simplest explanation I can give is entropy. Everything in the universe is in a state of decay. Eventually the thing decays so much it breaks down.