Ads are a recent innovation on top of existing web standards. A new generation of changeable scripted ads had emerged and this was not compatible with XML. The old generation of ads was still compatible but not scalable, similar to how podcast sponsor ads work (immutable after publishing), and so did not get much traction.
Podcasts ads are definitely mutable after publishing. Dynamic ad insertion has been a thing for years. If you download an old Stuff You Should Know podcast, you will get a new ad even based on where you were when you downloaded it.
