Why do most phono preamps lack XLR input even thought cartridges are naturally balanced?

In the experiment presented, and the "concept" for shielding presented, their is an external magnetic field noise source. As the shorted loop is passive, and it's current is only due to the external magnetic noise field, the field it would create cannot be larger than the field already created by the magnetic noise source. It cannot effect the other wire more than the noise field would have already. It would direct the field away from the wire.

Perhaps it works, but I would think that it can do more harm than good in comparison to plain grounded shield.  High frequency noise currents induced in the signal wire flows on the surface (skin effect) - shield to ground.  I'm not sure replacing it with shorted turn will be better, but who knows.  I'm just skeptical, but if it works it works.

Oh absolutely it will do more harm than good. It acts as a shorted loop to the signal wire, which means a parasitic load on the signal wire, bigger than anything else one could do.  It also provides no electrostatic ground. It is a terrible idea .... some "cable" guy came up with it :-)