I don't think it's a matter of reliability: most aspects of the Internet infrastructure are perfectly reliable. I think it is mostly a question of social convention: most of the people posting these ads don't think of the Internet first, and most of the people who might be interested in the ads aren't scouting the Internet. Part of the reason for both of those factors is age: the Internet is not nearly as pervasive among older folks (or in communities outside the Bay Area, for that mater).

Another reason is that the Internet is global by default, not local (with a few notable exceptions, like Craigslist). At least right now, there just isn't that much to be gained by putting local notices on the Internet: there's an incremental advantage, but it's not the sort of obvious, game-changing advantage that drove the adoption desktop publishing, for instance. That might change as technology improves (for example, being able to show highly locale-specific ads to someone based on the location of their mobile device).