Having to rewire a 59' 172 with some very tired/frayed stock.
taking the nav lights off, I noticed none of them had a ground circuit or a shielded portion connected to the airframe.
the landing/taxi lights did, the strobe beacon did, but not the navs.

Was told I had to run twisted pair shielded wire for the nav lights but was wondering if they were never grounded (or appeared not to be) then why would shielded wire be required now?

please explain as if you're talking to a Labrador retriever.
Thanks!

You must have the metal wing tips still. If you go to fiberglass tips you'll need a ground wire for the lights to work. I do prefer to have a ground wire from the light to the end rib as it does help eliminate some of the grounding issues in those lights. I don't see the need for a shielded wire running out there. If it were me and I was oulling new wires in anyways, I'd pull smaller wires in and put LED nav/strobes in and ditch the beacon. But that's just me.

They’re grounded through the screws into the mounting plate.

Issue 1 is that a 60+ year-old riveted airframe isn't the best conductor.
Issue 2 is "ground loops", a situation where the same circuit has multiple different ground paths/potentials.
Issue 3 is that a future upgrade to LED lights has a good chance of exacerbating anything related to issue 2.

These planes weren't built for intercoms or headsets, and they cut costs/weight wherever possible. These days we fly around with stereo intercoms and ANR headsets, electrical noise gets annoying fast.