The time it takes to browse each feed and to go to the next builds over time and can be tiresome after a while. Plus sites usually don't allow hiding seen posts (e.g. Twitter), and the ones that do are usually slow at that (e.g. Mastodon). And also, RSS feeds allow having a local backup of who you follow, without the risk of shadowbans (e.g. Youtube silently unsubscribing you) or the site you had those you follow go dark overnight (e.g. kbin.social). And lastly, it helps controlling just how much clogging there is in your feed - after all, people only have so much time on Earth.