They can be.
The cheapest RCA cables are coaxial and use 1 center conductor plus a shield.
XLR wires are balanced and shielded which needs 3 conductors. 2 signal wires and one ground which is almost always a shield.
The fancy construction RCA wires use the same XLR wiring described above. The 2 signal wires are used for the center and ring but the shield is connected only at the soruce end.
