- they are developed by hipsters who were in elementary school when RSS was in its heyday and don't know what it is.
- sites operators want to human users to interact with their site specifically, and not be able to skim the highlights through a remote interface. They want users to view all of the content (including advertizing) in its fully rendered form, and not some clipped version that has been commoditized and aggregated.
Basically RSS is a form of voluntary self-scraping. Through a modern perspective, RSS looks like "robots will scrape the site anyway, so why not do it yourself and offer the scrape as XML". Webmasters today deal with issues such as, for instance, Google clipping their content and incorporating it into search results, so that users do not "click through" to the site. When you offer RSS, you're basically doing sort of the same thing to yourself.
But, of course, RSS is great for end-users, just like a Google result that has the condensed info right there.