Faraday cages work not just because they are metal but because of the mesh. A steel box is not a Faraday cage.
The mesh is important because of the electromagnetic induction in the individual wires in the mesh (Faradays' law of electromagnetic induction, which is why the cage is named after the man).
Essentially, when a magnetic flux passes through a conductor, it induces a current in a predictable direction based on the flux change. The cage causes the flux passing through it to induce currents in multiple directions at once, which creates interference.
Also, these induced currents also create their own magnetic fields, which induce other currents, and so forth.
I don't think steel ships have their antennas mounted externally for this reason, because large fiberglass yachts have their antennae mounted externally also.