You don't have to exercise your copyright privileges to protect them in the US. You have to for a Trademark. Not exercising them can limit or reduce the damages you can claim though, if memory serves.
The major issue is that somewhere along the line the concept of intellectual property was invented (or perhaps imported from civil law systems) and gained enough traction that most people in the world think it's a real thing. The people who wrote the original copyright laws in the US and UK would be appalled it the current circumstance.
Copyrights and Patents are government granted monopolies. Like any monopoly, they're inherently bad. That said, society correctly feels that writers and inventors (and by extension musicians, artists, etc) not being able to make a living from their talents and ideas or having to depend on patrons, along with the benefits to society of public access to art in all it's forms and to advances in science and technology is a worse situation.
So we, via the government, grant a temporary monopoly to the creator to be the sole distributor of their work.
It is explicit in this that once that temporary period is over, the work is free for the use of everyone.
40 years or the life of the author, whichever is longer for actual people, just 40 years for corporations. No extensions.