For my part, it's pure laziness. There are a million different things worth writing about. And even if there were only one, there's a million different ways to write about it.
But if my mind doesn't feel like exerting itself, it will divert itself in any less mentally taxing direction it pleases.
It's more a matter of focus and discipline. If I have "writer's block", it's a simple indication that I'm too lazy to think. YMMV.