That is true, initially with extensions past the creator's death. But as you also point out, it is very much rent seeking behavior by companies.

However to understand why we even have copyright, you have to know your renaissance European history. Prior to copyright/patents, you had the guild system. Knowledge was hidden, obfuscated, and destroyed. Those whom wanted to share the secrets of the guild were imprisoned and/or killed.

Fast-forward to the creation of the USA - most of the people here were second sons whom had little to no claims on European riches. They also knew of the guilds, and the danger they have. So, they posited copyright, trademark, and patent. It wasn't cause they were the best tools, but they were better than the guilds that preceded them.

It's about time we revisit the idea of copyright, patent, and trademark and how they apply in the 21'st century.