Because so far it appears that the non-randomness falls off sharply at macro levels.
i.e., so far there seems to be no support for the idea that the universe is actually chaotic in the mathematical sense from the quantum level to the relativistic one; Brownian motion doesn't change the weather.
(I'm speaking specifically about the laws of physics in our geological era; the physics a few microseconds after the Big Bang would likely be different in this regard.)
I am not a physicist, so I'm speaking outside my realm of expertise. On the topic, Hawking suggests the following:
"Quantum physics might seem to undermine the idea that nature is governed by laws, but that is not the case. Instead it leads us to accept a new form of determinism: Given the state of a system at some time, the laws of nature determine the probabilities of various futures and pasts rather than determining the future and past with certainty."
It's an interesting idea, because to me it suggests that if one were to rewind time and play events out again without changing anything, subtle things may occur differently. This would lead to larger time changes over a larger period of time. The idea seems strange, but then, so do many things on the quantum level as they are completely outside our traditional experiences.
Apparently no consensus has been reached on this topic by experts in the field. I won't try and convince you, just present my interpretation.
