You most likely just don’t realize that your podcast app of choice actually gets its list of podcasts from Apple. That’s the authors point of Apple hosting it for free for everyone.
i recognize art19 as an amazon thing, and megaphone as a Spotify thing (or a thing which Spotify acquired?). i have no idea what any of the others are associated with. is one of those websites run by Apple?
Most (all?) feeds in there are not hosted by Apple. Many non-Apple podcast apps use this to power search and discovery features.
I do not know what you are talking about here. Apple has a podcast index. Podcasts do not need Apple's podcast index for anything. You can find a feed by going to the podcast's site and clicking on it, or googling the name of the podcast + rss. Or in any number of ways, really. Apple is one of many podcast indexes.
So no part of a podcast is hosted by Apple, unless the podcast is actually an Apple podcast.
Your question is the same as: "Why do you care if $YOUR_SEARCH_ENGINE (google, kaggle, duckduckgo) hosts a list of websites?"
The last search engine to host a list of websites was Yahoo!, and nobody did care.
The UX on the latter side is clearly superior.
in the past i've subscribed to a dead feed this way i think because the author changed who they syndicate through. i assume Apple's directory is maintained in a way that largely avoids this (simply by it being the canonical directory).
You could, but I suspect most people search in their podcast app. Which is where apples directory comes in.