Because they're too restrictive. They're also not something you have too much control over. My site started out with only my resume. I then added photos because why should Facebook benefit? Then came some work stuff I wanted to publish. And then I got into radio control and I posted up builds. In between all of that I play around with some JS ideas/apps/databases.

Maybe Medium and Quora are enough for some, but for some they're just a sandpit when what you really want is a warehouse with a bandsaw, drill press, 3D printer and coffee machine.