The prices of appliances has dropped dramatically in the last few decades.
Building a long-lasting appliance often requires even marginally more expensive materials - many of which are highlighted in this article.
If consumers are primarily price sensitive at the time of purchase, they may choose the $50 cheaper appliance even if it lasts 40 years less. What I mean to suggest is that price sensitivity at time of purchase often trumps long term gains - which is why you often see things like payday loans being used. When a person is paycheck-to-paycheck they probably aren't thinking about their financial position in 50 years, simply out of necessity.
Another reason might be that the Nokia 3310's of the world have been basically prototypical products - they are overdesigned. When the user has a perception that a phone is fragile, what's the threshold? We might not need a phone that can be run over by a truck to think a phone is tough. If the split of material cost -> price point and consumer perception is optimal at a certain point - companies will try and design to that point.