It's not just that jerks exist. It's that this "we welcome anyone who doesn't need a CoC to behave" is functionally equivalent to "we welcome jerks."

It's true that you can't just throw together a CoC and declare the problem to be solved. But there is value in writing down some ground rules. The purpose is not to "script" enforcement, it's to have something concrete you can point to. Having a CoC that says "no personal attacks" won't stop personal attacks, but it will let you very quickly shut down anyone who comes back with something like, "you just need to have a thicker skin."