This is simply not (always) true. Spotify injects ads for Podcasts even for paying users. YouTube has tons of videos with adds built-in by content creators.
this is an incorrect analogy - the platform is not showing ads but rather the creators themselves.
Yep, and a lot of the streaming services listed also inject ads for their own shows into the "ad-free" tier's content (before it begins). Plus ads on the home-page.