This is not necessarily true. It's just that this is the most likely time for weaknesses to become apparent, because this is when the computer system undergoes its greatest stress. Once manufacturing defects are weeded out, it is highly likely that a computer will run for quite a while before wear causes a failure. Power-on hours 4. Heat is a problem because electronic components may burn out if not cooled sufficiently.
Excessive heat can damage any component, especially physically moving ones such as disk drives.
Power supplies are sometimes a bit erratic for the first few microseconds when first turned on, resulting in initial power surges. The various wear factors have the greatest impact during power-on hours. Monitor phosphors and filaments eventually burn out. Humidity, salt, and pollutants can corrode various parts. Simple aging can also have an effect, although this is very small compared with the others.