All of these secondary factors (preparations, etc.) can themselves be regarded as "random" or at least unknowable in their extent and their effects. Much like lifetimes of individuals -- having some clear predictability on an individual basis, but in aggregate, well-modeled as random variables.

To be a bit more abstract, just because something is "random" does not mean it cannot be characterized with a distribution, expected life, etc.

The philosophical/mathematical effort at quantifying randomness has been called "The Taming of Chance".