It's not about who contributed more content to the final product, it's about who has permission. Game creators give streamers permission (sometimes just implie

It's not about who contributed more content to the final product, it's about who has permission. Game creators give streamers permission (sometimes just implied permission, but with more recent games it is more explicit) to share streams of their games. The streamers are not giving anybody permission to re-upload their content. The video game company doesn't automatically own the derivative works that they have given permission to be created.