Why does computer slow down over time

Why do all Window systems slow down over time? - Forums - CNET
Also does this problem exist in other computers not running Windows, or issue I experience exclusive to Windows systems? Any explanation as why this happens would be grateful. And if there is method to prevent the slowdownsovertime, it would be a bonus to learn about!

What causes Windows to slow down over time? Is it registry... - Quora
Whydoes Microsoft Windows slowdownovertime? Now of course Windows does not magically slowdown: there are reasons, and it’s not a single reason