You are making a lot of assumptions about what someone wants/intends when they contribute to open source codebases. If an author chooses, for example, the AGPL, I think they clearly had a different intention. Like it or not, not everyone wants to dedicate their work to the public domain.
GPL code is open source but what you do with it also needs to be open source as a condition of its use. Will CoPilot inform developers if suddenly the code suggested requires them to re-license their software?
Every large successful open source project I know is explicitly not in the public domain/licensed CC0. I understand that there are some people that are very against copyright/intellectual property but you surely must interact with a large number of projects/people that disagree.
