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Because scarcity was a real issue back in the day perhaps?

That's all I can come up with is people had a fear that there wouldn't be enough so they began to hoard as much as possible instead of taking the other route of sharing.

Human wants and needs are infinite while resources are limited. In addition, different people have different skills. We trade, barter, and share so that we can all take advantage of our relative strong suits. I am not good at making clothing so I have to acquire clothing from someone else. Since clothing is hard and unpleasant to share, I purchase it.