The "why" is that distribution is always cheaper than production of creative works. In other words, people who make new works are stuck with all the costs, while people who sell copies get all the profit. This also tends to be an asymmetric relationship: creators tend to be individuals while publishers tend to be large corporate enterprises with the ability and know-how to bury you. Copyright exists to force publishers to negotiate with creators that would otherwise have no leverage[0].

The idea that copies are supposed to be scarce is an invention of Disney marketing, and I'd argue contrary to the spirit of the law. The stated purpose of copyright is to increase the amount of works on the market (by giving creators some negotiating leverage with publishers), not to make individual copies more valuable. That's just a side effect, one that is supposed to be bounded by limited term lengths[1].

[0] For those wondering, this is also what drove copyright harmonization over the last century. Any difference in law between countries creates an avenue for publishers to find a way to cheat creators. Before the era of international copyright it was quite common for publishers to race creators to foreign markets, since translation was not yet an exclusive right of the creator and most countries did not recognize each other's copyright interests.

[1] Please stop laughing. I know life+70 is practically forever.