But unless you own a datacentre, you are always using someone else's platform, whether it is a cloud provider, a colocation or anything in between.
If my domain registrar, my DNS provider, my hosting company, ... go "bad" I can just move and my readers won't notice. Even if you are on hosted wordpress etc, you can archive your site as is and move, preserving the URLs. That's not the case with Medium etc where your content lives under their domain.