Why don’t creators both publish to YouTube but also publish somewhere else for archival or public access reasons, to help keep content available for outside walled gardens? Is it just not important to them? Is it hosting costs? Missing out on ad revenue?
I just mean some kind of public service like one of those archive sites. So they would place it into YouTube for revenue but also these other places so there’s a way to get the videos without Google being a dictatorial overlord.
There's no incentive for them to do so. It reduces their ad revenue, while costing more money to host it. That said, if you are a creator and you do want to do it, Peertube is a good option because it uses torrent technology to reduce your hosting costs.
Youtube pays them per (ad) view, and also recommends the video to more people based on how many people click on it. So giving people another way to watch it will decrease their revenue and audience.