The scarse resource is not that song or that movie. It's the time of the people that created that song and that movie. That's is part of the cost of production.
This is similar to there are millions of houses but only a few in the most scenic places. Want one of them, pay more. It could be the same for the most popular songs and movies. The time of their creators is more valuable than mine. When writing software probably the other way around.
Of course they don't sell songs and movies like that and creators sometimes only get peanuts. I'm not a very good Devil's advocate.