Instead of hoping every website operator in the world does something, it seems far more scalable to centralize efforts into a youtube-dl style repository so anyone can host their own website-to-RSS gateway.
With email, you can track who reads the mail, how often, what times, etc. You can tailor recommendations and content to the specific person
RSS is just a generic feed that's the same for everyone. Great for consumers--less useful for people trying to optimize monetization
Implementing RSS is extra work, thus not everyone is going to implement it. Even if it contributed to the bottom line. It's the same reason every website that publishes information doesn't have an email newsletter.
Since it's a consumption optimization, it makes sense for us, those who may want to consume a given website, to implement it (while we wait around for utopia).
But for trickier sites, or ones that are popular and need more maintenance, projects like RSS Bridge and RSS Hub are worth a look.
It's a pretty simple protocol, even if I would have done things differently. For example, relative paths should be supported with some sort of top level setting called "root" or something like that. That way I wouldn't have to convert relative paths.
Atom feeds support xml:base explicitly.