-Slapping some ad network's code into your site is comparatively easy, and monetizing almost every other way is hard (i.e. "I just want to create awesome content and get paid for it, I don't want to be my own sales department, too, that's boring") -Even if you do monetize other ways, slapping some ad network's code into your site is, effectively, 'free money' on top of what the site's users are willing to pony up for -Users have been inured into believing and accepting that covering every possible square inch of civilization with ads is an unavoidable consequence of modern life. So much so that the absence of ads in something that isn't otherwise monetized is suspicious (there's no such thing as a free lunch)