Can somebody explain why exactly? Was there ever some large scale research with the outcome this was somehow the cheapest? Or is it a mere byproduct of the consumption-based society? I'm asking because I'ved used lots of different types of hardware (both computing and non-computing) and >90% lasted well over 3 years. It seems a waste of time/money to get rid of ot after 3 years? Which matches with SE's findings: they choose 4 years. Still not that much, but already 'better'
Possible that this attitude (along with many other factors) contributes to the far less developed startup cultures of many Western European countries.