Advertisers love to burn money, but they draw the line at not being able to verify that the spend did what was promised.
Let's not. Please.
With email, you normally use unique image and link URLs for each recipient, so you generally know who's opened the email and what they've clicked and can map that to their email address and whatever other information you have about them.
With RSS, you generally don't have any information about who's accessing the feed other than an IP address. It is possible to require users to log in and receive a unique RSS URL, which is what podcasts often do to give paid subscribers access to paywalled episodes, but that's not common for web RSS.
Even the injected ads idea was tried with companies like FeedBurner, later acquired by google.
If you're generating feeds on the fly with tracking metadata based on the requester, you can identify aggregators, and treat them equivalently to social media platforms where users circulate normal web links. You still get click-throughs to the underlying content from the end users, and you'll know the aggregator was the referrer.
Many podcasts use direct ad injection using metadata from the request to the enclosure link -- that seems to work well enough, and seems like the sort of thing that could be used for other content than just audio and video.
If RSS has been more common, I imagine the bigger RSS readers (bearing in mind one of them was from Google!) would also have standardised on other ways of tracking clicks and ad views and all the rest. There just weren't enough people interested in RSS to make any of that worthwhile.
I say this as a user of RSS and someone who publishes a (very sporadic) RSS feed. It's a niche, because most people don't want to curate their own feeds.