The exact number can be tuned but there is nothing wrong with being somewhat arbitrary. The age we let people drink is arbitrary. Why 18, why not 19 or 17. Its just a number picked to be good enough.

>Don't forget some people might take years of thinking about a story before ever releasing it publicly.

It applies as soon as you release it publicly. Copyright doesn't instantly apply as soon as an idea pops in to your head. It only applies as soon as you can prove you created it at a date.

> The age we let people drink is arbitrary

Maybe this was childhood conditioning but I was told 21 is the general age where your brain is fully developed so that drinking alcohol doesn't affect your cognitive growth.

Mid 20's actually. Some earlier some later. Even as late as their early 30s.

Doesn't copyright usually start from date of publication? It changes a bit for unpublished works, but otherwise a long gestation period doesn't eat into the duration of exclusivity. [1]

[1] https://en.wikipedia.org/wiki/Copyright_term#Length_of_copyr...

Isn’t cost of creation just one of many normal inputs to the marketing and pricing of every product? What’s special about copyright? It’s also true for, say, an idea about starting a company, but we don’t have laws to somehow adjust the potential return on investment of a company based on how long the founder was thinking about it.