I'd assume it's not specified as a upper limit but that it's just the highest voltage/current-pair currently defined.
On a more fundamental level, there's the physics of how much power you can pass through a cable/connector safely.
On a more fundamental level, there's the physics of how much power you can pass through a cable/connector safely.
Doing a quick bit of research there isn’t really any other option. Silver has 7% less resistivity than copper, but it tarnishes.
Aside from that, humanity hasnt discovered other options!