3 reasons... nobody has time to put their ads on specific websites - it has to be automated to reach thousands of websites/communities at once. Second no adver

3 reasons... nobody has time to put their ads on specific websites - it has to be automated to reach thousands of websites/communities at once. Second no advertiser trusts an individual website selling ads, unless there is tracking and reports and they know everything about the demographic of people who see the ads. Third - since advertisers are used to highly targeted ads and tracking... an individual website would basically have to rebuild all those tools in their personal system. That would be impossible without 3rd parties.