Documentaries also seem like a category where I care the very least about the difference in picture quality between 1080p and 4K.

I doubt that it’s related to bandwidth costs; those are likely minimal compared to the price of acquiring content. It’s https://en.wikipedia.org/wiki/Price_discrimination - a way to capture more money from those who get more value from the product. It might also be a little bit of https://www.economicshelp.org/blog/glossary/premium-decoy-pr...

Some youtubers are sponsored by this brand.