I reject copyright because I believe users should be able to modify their software and redistribute their improvements. Open source licenses are a tool I employ to give more users that right, not an end in themselves.
Why are open source licenses good? Because they make it easier for people to improve their stuff and share those improvements, or write new works based on existing works.
Copyright incentivizes many authors to undertake works they normally might not due to the protections copyright provides. Many authors (who freely publish their books under copyright) probably wouldn't be able to spend 3 years researching and writing a book if they weren't able to get advance money from their publisher.
One is free to use a liberal license or a more restrictive license, both are out there and available to creators.