Because “Indians” used to bury people this way as a form of torture, or test of bravery–only they probably didn’t really, at least not as much as movies might have suggested. Here’s a clip from the movie “Jeremiah Johnson” (1972), which was very well known at the time and probably inspired this strip: https://youtu.be/pYhlVR9GzjA?si=klSXoYG0m3ynJzE4