Obviously, many people were very unhappy with this idea that chance can rule physical properties. In response to this theory Einstein proclaimed "God doesn't play dice!" (Einstein was wrong).
If you flip a coin two times and get two "heads",
you can't conclude too much about the chance of getting a
"tails" on the next toss because your sample size is too small.
However, if you flip a coin a hundred times and get
51 "heads" and 49 "tails", you can conclude that the chance of
getting a "tails" is about 50%. Even though chance does rule
physical properties, physicists can accurately
predict the overall results of a large sample of random events.
In the case of half-life,
they can predict when half of the atoms in a given
bunch will have randomly decayed.