Partly, it's because they have not been able to resist the conditioning that the advertising agencies impose. I have a sister-in-law that does not go shopping just to get what she needs. She also spends time comparison shopping to figure out what the best deals are, and in the process she buys things that she doesn't need because "it was a good deal". If someone needs some potatoes, lettuce, cheese, a little meat, etc, and comes out of the store with a whole cart-full of stuff because the prices "looked good" or "it was lots cheaper than the lost time I was here", there are some false economies at work. Buying stuff that you don't want or need because you've bought into a store's roller-coaster pricing routines is silly, and it's stressful. My sister-in-law realizes that she's falling for the retailers' tricks, but she justifies her purchases by comparing what she paid for things to what they might have cost her if she had bought them previously. It's a sickness.