Why should two copper wires be twisted instead of parallel wires?

The network cable is the structure of four pairs of copper wires twisted in pairs, which is also called twisted pair. Why should twisted pairs be twisted with two insulated copper conductors instead of parallel? The outer skin wrapped with copper core is actually the insulator layer between wires. They are equivalent to a dielectric plate, and the high-frequency signal passing through the network line is > 10MHz.