Why would it be impossible for a government or large cloud company to control all the nodes in a QKD system? In that case, they could rely on the physics to det

Why would it be impossible for a government or large cloud company to control all the nodes in a QKD system? In that case, they could rely on the physics to detect eavesdropping. The problem for simple customers is that they can't be sure that the transmission isn't MITM'd somewhere, but that's only because they don't build and maintain the whole channel themselves.