alt Hacker NewsI think it is because humans spent thousands, tens of thousands of years not doing much other than searching for food and trading one physical object for another physical object.
The idea of trading something valuable for an abstract piece of software or paper is still not really natural to us, and is a learned behavior.
I buy almost everything with a piece of plastic that represents a company who's agreed to lend me money that represents absolutely nothing except the common agreement that it's valuable.