This map took at most 7 hours to implement. Now in my opinion there's two reasons most other sites don't offer this:
1-it doesn't come as part of any package (ubb, vbulletin, ezboard,...)
2-most car site webmasters don't have the required skillset and/or resources to implement it
So how is something like this done?
<small>First of all, you need a database of all zip codes and postal codes with associated city, state, long and lat and bulk bulk import this into your favorite SQL database (15m). After that you must hack away at the UBB code so that you can recognize people's logins into the board and use them in your registry (1h). Crunch away a few hours of PHP code gets you the registry to enter your info(2h). I use an SQL database because it would suck having to reinvent the will (code the sorting algorithm, etc). Another hour of code gets you the long/lat and some creative SQL code gets you the registry list sorted in growing distance (for the user that is logged in) (2h). Then you need a map of north america with specific endpoints in long and lat. Precision is important. I used Mappoint 2002 to get this map. Some more php code using DHTML puts the little smileys (representing each user) over the map (1h). The thing is, if you aren't 95% familiar with all the above technologies, it could take you 10,20 times longer to write the code.</small>