It is more to do with the fact that vast majority of people aren't going to bother subscribing to an RSS feed. I am on a freelancer slack group that supposed to have a RSS feed for jobs. The feed is often broken for weeks because most people don't use it.

Even when it isn't broken the display output is broken in Thunderbird because the dev isn't going to bother checking Thunderbird as many people don't use email clients like that anymore and instead use webmail.

I never have used RSS that much as normally if I want to check for new things on a site, I will just go to the site and look myself.