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?

Where else should they be publishing to? And who is going to pay for this service?

Don’t forget - most “content creators” are not technical - self hosting is not an option.

And even if it were - it costs money.

For a lot of creators, YouTube is the internet

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.

LTT kinda do, but they're the exception, not the norm