Authors can buy those services directly but why bother trying to source and vet those things yourself? Some might, and I know the HN crowd is very much of the "I can do everything" mentality. But most people will be happy to pay an intermediary to do those things that are outside of their wheelhouse.
The intermediary does not have to do it for a percentage of the sales though, these are jobs that are charged by the hour normally. Most freelancers do not want equity in your book, they want cash, so a sane intermediary would just charge a fixed fee.
