In the first place, because almost anyone can afford to buy a used one – as opposed to what it costs to buy a good used car or truck these days. The latter has become almost an extravagance and those are tough to indulge before things get really bad. Many people are understandably reluctant – out of necessity – about buying a "survival" car (or truck) they can't really afford, especially without selling the car or truck they already have.
But you can readily find a good used motorcycle for $3,000 or less – which almost anyone can afford – if you focus on the kind of motorcycle that makes the most sense for the purposes of this discussion.
A minimalist bike.