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I'd just have lots of rules and variables.
Now I agree that complex combat can be fun, the scale of the combat under discussion does not lend itself to such detail.
So although a detailed combat system as you described might be enjoyable in a smaller scale game, in a game like Civ, where the Squares on the board represent hundreds of kilometres (or more), such detail is not really feasible.
An easy way to simulate such detail existing, but without actually going into it, is to use a random generator.
At the coarse level of detail that Civ (and other games like it) have, this level of detail does not exist, so the only response is to use some other system and the easiest is that random number generator.