This doesn't remove the ability to promote and share your content elsewhere. Content creators really can have it both ways. Publish on your own site and link to, share or duplicate that content on Facebook, Instagram, Medium, Twitter, RSS, Soundcloud and any other service that may be relevant to your audience.
Ease of use. Plenty of folks don't have the skills to set things up and keep them humming along, nor do they have the funds to play someone else to do it for them. So they hand over a bit of control in exchange for a platform that's set up and kept up to date.
You may argue that they're giving up too much control, and I'd agree with you, but that's not how most (non-technical) users see things.
I know one of them personally, and I think in her case, she may have had a few hundred people following her art blog, while on Instagram, she quickly amassed over 100,000 followers, so when she stopped updating the blog, she lost less than 1% of her fans.
It comes down to people knowing how to follow Instagram/Facebook accounts, while they haven't even heard of RSS.
Again, I'm not recommending giving up those channels, but betting your content on a platform that will be gone/unpopular at some point (they all come and go) while abandoning the constant, central presence of your very own website, regardless of how small the following may be, seems short sighted. This has nothing to do with RSS.