You can think of shorting as betting that the stock will go down. For example, Alice can write on a piece of paper: "Anytime anyone wants to sell this paper back to me, I'll pay a sum equal to the price of GME at that moment". Now if Alice sells the paper to Bob, Alice is short and Bob is long, even though none of them touched any stock.

Of course if many Alices do that, that will push down the price of GME, because it's market-relevant information that many people think GME will go down.

The real problem is counterparty risk. What happens if Alice runs out of money and can't pay up, or there's a run on all Alices at once? So a system of guarantors is needed. But this is far from the simple argument that "selling a borrowed car is stealing".