RSS makes it very difficult to measure the performance of an ad. Most players in this space wrap a publishers hosted podcast link with their own tracking link, and the main metric of engagement is a download. Downloads themselves are a pretty crude measurement and don’t hold up to something like minutes watched/listened.
Placing ads into the audio directly also just isn’t as performant as streaming it into the media like in video. Publishers and advertisers want more fine grained control over where the ad is placed, when it’s placed, and how it’s consumed by the listener. Podcast ads currently don’t deliver on that.