You could flip the question around & say, “The fact companies like this, which are mostly rational companies that have ample time & resources to research contracting or outsourcing as alternatives, and yet are choosing not to must mean the economic value of contracting and outsourcing is less than relocating long-term salaried employees.”

In other words the question is, “Given that contracting and outsourcing are deemed to be worse options by rational actors, what reasons make this true?”