For people who move every few years, whose requirements change (from solo to 4-person family), and so on, the current situation is pretty ideal.
Spend less money, change models more frequently to fit your changing needs, and often it's even cheaper to buy a new one than to get it fixed -- which for consumers is amazing! (Because repair costs certainly aren't getting cheaper.)
Of course, the negative externalities on the environment are pretty clear and potentially horrific. As well as what it means for cultural values where more and more things are disposable.
But that's the answer, that's why they don't last 50 years -- because consumers actually prefer something that lasts only a few years at 25% of the price.
[1] http://www.aei.org/publication/the-good-old-days-are-now-tod...