And people "know" not to steal. Yet we need to have laws so we agree on rules and boundaries. You can think of it like a list of rules agreed on early, so the project leaders don't have to go through the arguments about what's accepted after someone crosses one person's boundaries, but not another's.
The core principle of laws is that they are common and enforced equally onto every citizen. Laws and enforcement are transparent and has check and balances.
The most common objection to CoC is that they are not enforced equally. Different classes of people get treated differently and enjoy different rights. The enforcement have no transparency and no check and balances.
To take two recent well known examples, we had during the last year the stackoverlfow controversy over the new CoC, and wikipedia ban of an admin over CoC. In both cases there were major issue of transparency and selective enforcement. In the wikipedia case there was also problem with lack of check and balances. In the stackoverflow case there was also voices raised about other common rules now being unenforced in order to comply with the CoC creating a situation where different people get enforced by different rules.
Laws can be good. Rules and Boundaries can be a positive thing. They are not all good, not all positive. It depend. Without careful process and system for fairness it is very likely to turn bad as the above examples illustrates.
You're raising good points regarding consistent enforcement and transparency, but I think they're orthogonal to the existence of CoC. You can have transparent handling of issues without preexisting rules. You can have rules with bad/no implementation.
Sure, some CoC rules may be not great. But having some rules written down at least indicates leaders thought about the problem.
