Because they can be stolen and in many cases they can be quite easy to steal because people don't lock them up. It is a way to make money for some without much work. Plus as long as there is a market people will keep doing it. If people stopped buying stolen bikes and equipment there would be less theft and if we could provide people with good jobs that pay fairly and provide people with quality healthcare and housing without putting them in financial ruin, maybe just maybe we wouldn't need to steal much if at all. I mean one could go much deeper than that but if we all take care of each other we may not need to steal from each other?

Stealing can also provide some cheap thrills and people love excitement. Maybe provide people with more accessible thrills and that could also help.