It would have to be a massive car park to equate to the exposure on the internet. IMHO
When your driving around it's by chance that someone might like the look of your Landy and take a note of the plate to find out where it lives, or just follow you home. On a forum we provide a concentrated source of information. It's a lot easier to sit with your feet up, beer in hand, looking through forums for decent vehicles that you can find the location of, than driving around supermarket car parks looking suspicious.
Sophisticated crims find that too much like hard work. They use bots to automatically scan forums and social media, they return data about anything they are programmed to collect. eg holidays mentioned on people's facebook pages