> Why on earth would they not use their own platform to present their content?
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.