You're absolutely right that artists can't stop themselves from creating, but I think that a reasonable amount of protection still does encourage more works.

Many works require a good deal of investment and time and if people had little to no chance of making money or breaking even on that investment a lot of works wouldn't get made.

Another nice aspect of copyright law is that it establishes where a work originated. Authorship gets lost in a lot of the things we treat as if they don't have copyrights. For example memes, or the way every MP3 of a parody song on P2P platforms ended up listing Weird Al as the artist regardless of his involvement. It also happens in cases where copyright really doesn't exist like with recipes and as a result we don't really know who first came up with many of the foods we love. A very limited copyright term would more firmly establish who we should thank for the things we enjoy.