Why does a loose pedal destroy the crank threads?
Once loose, the pedal spindle will describe a cone as the crank rotates. This concentrates force at certain points in the crank threads, leading to damage.

Top image shows the pedal spindle in the threaded hole in the crank. The threads are fully engaged and tightened, and forces are distributed. Note there is space between the male and female threads (somewhat exaggerated).

Bottom image show what what happens what the thread is loose. The spindle can move around in the threaded hole in response to changing pedaling forces, and forces get concentrated at the red dots.