Conductors don't care about voltage, insulation does. The type and amount of insulation is dictated first by voltage (safety), then by other factors such as durability, flexibility, temp etc.
Insulators don't care about current, conductors do. The size of the conductor is dictated first by current (safety), then by other factors such as length, temp, flexibility, etc.
So even though it just happens that large conductors also tend to have thick insulators, it's not guaranteed that they're appropriate or safe for a much higher voltage level.
Having said that, I have put over 400v of solar through 'landscape wire' that says 30v on the outside, with no 'detectable' leakage. Now I have access to an insulation test meter so i'm going to zap it with 1000v and see what it does. But we do such things at our own risk, and don't recommend anyone else to follow! Following the actual voltage rating of the wire is absolutely the safest thing to do.