Materials vary. Situations vary. History, performance, results may vary. Whenever you know something, or see something, or remember something, or do something you’re experiencing a bit of slop. Nothing is mathematically certain.
This is one of the reasons games of chance are so attractive to us. We deal with small randomness all the time, and bringing it out into the open and freely admitting “I have no idea how this will turn out” is satisfying in a way. We hide and ignore the randomness in so many other situations that letting it out feels right. “It’s about time” we seem to say.
But chance isn’t everything. In fact, chance is only the lack of control on top of our skill. If there were no intent, no goal, there would be no chance. Ability is the cake, and chance is the icing. Sure, you can eat a tub of icing, but come on, I’m trying to make a point.
Slop and chance can take many forms. Raw materials are often of varying quality. We pick through boards, looking for the best ones, because they are not all the same. But this is just the point, we have the ability to discern between the good and the bad. In real life, we can use our faculties to weigh the odds in our favor. We smell the milk before pouring a bowl of cereal. We read up on the market before investing. Yes, there is a great deal of randomness in the world, but we fight against it. Gathering, identifying, and hoarding are all ways of narrowing the error bars. Well crafted games reward this insightful behavior.
Games of pure chance take advantage of our feelings of powerlessness. They eliminate all methods of separating valuable from worthless. Slot machines, lotteries, and their many many electronic analogs all sing the same song. “The best you can do is leave everything to chance. Hard work is worthless. Discernment is pointless. All efforts come to naught. Why bother?”
Now, if you believe in Luck, or Fate, or a Power that governs probability, well and good. I do as well. Go and appeal to your god, and I will appeal to mine. We will see which altar is consumed. But these are other matters. Do not equivocate substituting another’s will for your own with “trusting Fate.” If you have no will to win, why play?
Games should reward finely tuned insight into the system of rules which govern them, not dedication to a bet (winning or losing). Games which mix skill and chance are valuable to learn how to accommodate shifting fortunes, but games of pure chance say something profoundly false about the universe.
EDIT: Keith Burgun has some good points on why randomness should not be in games at all.