I think it's just because people's jobs are becoming more specialized around the world. At some point, the time and money involved to save the amount makes it much easier to buy something from a store, and having it picked up or delivered is better. Cooking takes quite a bit of time and I don't like cooking as much as I used to.
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This is part of it and also why there is so much volatility. Entire sectors can be put "out of work" because they are no longer required, but they can't do anything else.