I see this point often - and similar ones about entitlement - but my reaction at the time to both Mark and _why was one of concern: they were people whose work I absolutely appreciated, and who I therefore appreciated as people. When they vanished abruptly I was concerned for them on a personal level, and naturally I wanted to find out if they were OK.

The unfortunate consequence is that concerned checking in doesn't scale well, and is indistinguishable from mob action to the person being checked in on. (Which is how it played out, if I remember right). So even though the person certainly doesn't owe anyone an explanation, it can be in their own best interests to provide enough of one to curtail concern.