Thinking about going digging for gold in the dumpsters around your town? Start with the FAQ at Dumpsterworld, the online community for skip-spelunkers:
There's many reasons that perfectly good things go in the garbage. One of the biggest reasons is business practice. Remember that businesses are there to make profit. Goods that are overproduced, don't sell, need repair, or take too much space and maintainence, are unprofitable to keep. It can also be unprofitable to sell them below cost or give them away free, so they go to waste. Wasting goods helps retailers profit if people might otherwise pay for new ones, and producers profit when more get made.
Waste is a regular result of doing business. Consider how the government props up agriculture and stops it from having a depression, in years when the grain market is saturated. It buys excess grain from farmers, takes it off the market, and lets it rot in warehouses. Farmers still get paid for it, and then they can sell their regular supply without the price dropping below cost. In our system, competing suppliers are always producing more stuff than they can sell, and the excess goes to waste.
Business policies enforce waste. Department stores toss products for cosmetic damage or an open package. Offices toss equipment when they upgrade. A college might toss last year's furniture for new, because it has to spend money so next year's budget doesn't drop. Groceries toss sealed containers of food when it expires. Expiration dates are planned for selling, keeping in mind that a consumer will have days or weeks more for use of the goods.
Relative worth is another reason why good things go in the trash. Wants, needs, and usability change between people. John Moneybags dumps his sofa because it doesn't match the wallpaper, Jane Englishmajor trashes a pile of books because they're too bulky to carry home for the summer, and Joe Bluecollar throws out his TV because he doesn't have time to fix a bad wire.
(via Beyond the Beyond!)