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The original duration of copyright was a flat 14 years, with a single additional 14 year extension if the copyright holder applied for it. So 28 years in total. It turns out that after 28 years the vast, vast majority of copyrighted works have already earned essentially all of the money that they will ever earn. Most of them go out of print forever before that point. It's only a rare few works that end up becoming "classics" and spawning "franchises" that last beyond that point. We're sacrificing the utility of the vast bulk of what should be in the public domain for the sake of making those occasional lucky hits into cash cows. There's a great paper by Rufus Pollock, Forever Minus a Day? Calculating Optimal Copyright Term, wherein he uses rigorous economic analysis to calculate that the optimal duration of copyright for generating the maximum value for society is 15 years with a 99% confidence interval extending up to 38 years. So remarkably the original law hit the right duration almost exactly through sheer happenstance.

In an earlier paper he also determined that the optimal duration of copyright actually decreases as it becomes easier to distribute work, perhaps somewhat counterintuitively.

Also, why is it that different things enter the public domain at different times? For example, I learned of a movie from like the 1970s called The Last Man on Earth that is public domain, or at least I'm guessing it's public domain, and yet it's from the 1970s, where this is talking about stuff from 1929. I obviously know some stuff is made and released immediately into the public domain, such as open source software, etc. But I wouldn't figure a movie would be like that.