Bit of a long story...

Some forms of intellectual property require registration. For example patents. Patents are supposed to encourage technological development by allowing inventors to monetize their work. There's a lot of justified criticism of that system but, on the whole, it seems to have worked.

Originally, US copyrights worked in exactly the same way, for the same purpose. The requirement to register for copyright was dropped in 1978. However, registration still plays a role in US law for some legal purposes.

So what happened to copyright?

Europe developed a different copyright tradition, in the 19th century, while it was stilled largely ruled by oppressive autocracies. The monarchs of the 19th century were not the overpaid figureheads that still exist in some countries.

Copyrights today last (usually) until 70 years after the author's death, while patents which underpin tech progress last (usually) only 20 years in total. You can see that this is very different. That copyright revolves around the death of a person shows how it is a personal privilege, as were normal in aristocracies. The purpose is to enable people to extract money without any consideration for the interests of society as a whole. It's about rent-seeking.

Nowadays, US content production (Hollywood, etc.) dwarfs that of Europe. The better copyright laws of the US may have something to do with that. Although the US has gradually shifted over to the rent-seeking European model, there are still some advantages left.

As the content producers in the US grew, the US gradually switched over to the rent-seeking model. I think this is largely because the content producers also gained more lobbying powers.