For a number of reasons, international itineraries are more complicated than domestic and generally domestic agents do not have the experience and knowledge to deal with the complexity of international itineraries.

International agents have more senority and have worked their way up the system. They are trained to handle the international desk. They are more knowledgeable about issues such as routings, complicated fare basis, geography, intricacies of code sharing, etc. It really can be challenging.

As soon as a domestic agent sees you are travelling internationally, they will transfer you to international, even for something as simple as a seat assignment question. Efficient? In that case maybe not, but the fear could be that you willl start with a simple question and then begin to ask questions beyond their competency and require a transfer anyway.

The airlines do not want a domestic agent providing inaccurate or incorrect information, which could result in a very unhappy customer and perhaps even compensation if they have clearly misled the passenger.

Domestic is domestic. International is international.