I think people simply don't appreciate that it's an easily caught crime. Most illegal things that one might do won't ever be caught, eg you break into someone's house and it would take the police to bother investigating and then put enough effort into that one case. Or to stay in white collar you might forget to report some income, how's the tax man to know unless you are already in focus?

With insider trading the investigation is a DB query. It costs very little to look at all the trades for everyone around announcements and see who was acting suspiciously.