Why is selling what one does not own forbidden? I want to understand the reason.

The reason for the prohibition of selling what one does not own is that it falls under the category of bay' al-gharar (uncertain or hazardous sales). This includes selling a commodity before taking possession of it, selling someone else's property contingent on the owner's approval, and selling something that is not guaranteed. It may also include selling something that is absent from the seller at the time of sale, or selling something on the condition that it will be purchased for the buyer from its owner. This is gharar because its acquisition is not assured, such as selling a runaway slave (abiq), a stray animal (sharid), or a bird in the air.