They don't add any abstraction or, if you will, they don't abstract over code that's not with operators and/or overloading. There is only ever a single correspondence. That operators don't add any expressive power is, I hope, easy to see, as they're just notation. That overloading doesn't add abstraction you can see from the fact that there is only one "implementation" of resolution at any call site (if there were more than one, then it wouldn't be static dispatch). Overloads offer shorthand, not abstraction.

Another way to see what's abstraction and what's not is to think what you can change in one place in the program without changing another (because that's what abstraction offers: indifference to the different elements in the equivalence class it creates). But there is no change you can make in a program with overloads that will require fewer changes in a program without them.