I think one reason is that people are just bad at statistics. Chance of materialization * impact = small. Sure. Over a short enough time that's true for any kind of risk. But companies tend to live for years, decades even and sometimes longer than that. If we're going to put all of those precious eggs in one basket, as long as the basket is substantially stronger than the eggs we're fine, right? Until the day someone drops the basket. And over a long enough time span all risks eventually materialize. So we're playing this game, and usually we come out ahead.

But trust me, 10 seconds after this outage is solved everybody will have forgotten about the possibility.