...lightning strikes on aircraft were relatively innocuous...
In the air, general aviation pilots are steering clear of thunderheads. So, their airplanes are vulnerable to
cloud-to-cloud lightning. It's a couple of thousand amps, and skin effect acts to conduct the bulk of the damaging currents around the surface of the plane (assuming aluminum here). You can still get a fraction of the current into the interior of the plane, with various bad outcomes. Relatively innocuous.
An aircraft on the ground is a different story. Yes, the skin effect still works. But in this scenario, the plane is subject to a
cloud-to-ground strike. Figure 10,000-50,000 amps, over a few microseconds. Lots more energy. Another factor: the last few jogs in the path of a cloud-to-ground strike induce risers from the ground, resulting in a meet in the middle situation. Truly spectacular.
Just bad luck for the unfortunate aircraft. Yes, the wheel bearings are welded. Maybe, the skin effect has spared the plane's avionics.
The issue of grounding, or insulating, the parked aircraft, doesn't matter. A protective cage has to surround the plane to be effective.