I can't speak for others, but one reason why I'd use something like Medium is precisely to ensure that I don't manage anything by myself except for the content production part.

I have a long list of attempts at setting up my own site, but because it's mine I wanted to get it just right. And so down the rabbit hole I go: making sure my server is super-secure (because it's important hey I'll learn in the process!), finally trying out Caddy, using Ghost (node.js), building my own static site generator using React for the backend, which of course means updating and perfecting my Ultimate Front-End Setup.

And once that's done, I want to do right by the design, and of course make sure my html/css/js is impeccable and that it all runs on every browser under the sun.

Of course I love doing this and I learn a lot from it, but if my goal is to write stuff and put it online, this approach never really works. I'm happy it doesn't, because it means I still love what I do more than writing about what I do, but it's one very legitimate reason to use something like Medium.