The difference between domain names and let's say cars or boxes of paper, as you put it, is so obvious, I didn't think it needed saying. It's that (usable) domains are a unique and finite, whereas all your other examples are neither. You can't just wait for the next batch of domains to come out of the factory and buy them from their manufacturer (new gTLDs were an attempt at that and failed miserably).

It's the same problem as land. We have more and more people that need it, yet the supply is by definition finite. Hoarding it for yourself not only contributes nothing to the community, it actively prevents others from doing so.