Suppose you and I own competing seafood restaurants. Suppose you know I'm planning a very important event and shrimp is the main item on the menu. Suppose you also know that I have a legally binding contract with a shrimp supplier and would have a hard time finding shrimp for my important event if my supplier failed to deliver. Say you know the shrimp supplier has almost no assets and is basically judgment proof, and you know I would suffer damages well beyond what he could ever pay me back. And you really don't like me, and know that if I suffered financial harm, that would help your business. If you offer the shrimp supplier a large sum of money to default on his contract with me, have you violated my rights? Of course I can sue the shrimp supplier, but can I sue you?

Because your argument only works if contracts are *never* enforceable against third parties.