Facebook makes your content virtually inaccessible and discoverable from those outside Facebook. They are the opposite of the way things were on the open web. Medium "is easy" but they are gradually asking more and more of people visiting too. Twitter is a good platform for streams of thought but it is a terrible "blogging" platform. Having "a series of tweets" does not make for a blog. It is hard to read and terrible.

But you are right. People feel they don't need to self host because these things are there. The problem is you then no longer own your content distribution and are at the whim of them walling it off or shutting down.