This is allowed to happen precisely because advertisers have been conditioned on 30-odd years of broadcast TV advertising, back when that was the only mass-media game in town. The strategy on broadcast TV, whether in its heyday or even now, was essentially the shotgun approach: fire a shotgun into a crowded area, and hope to hit a small percentage of your targets (metaphorically speaking, of course!).
Web-based advertising can, potentially, allow for a much more sophisticated and surgical approach. But advertisers aren't used to thinking surgically. They still want their shotgun. (They call the shotgun "reach," or use terms like "GRPs" to attempt to quantify reach). That's why broad, sloppy metrics like total video views, or total pageviews, or total clicks, still appeal to advertisers. They're still working from the mental model that you need to fire a big spray of shot, and that you apply some sort of fractional discount to the total number of people you fired at, and that fraction is who you managed to get. It's a dumb, top-down, unsophisticated approach to advertising -- but it persists, and so long as it does, there will remain a healthy market for "viral" services.
Web-based advertising can, potentially, allow for a much more sophisticated and surgical approach. But advertisers aren't used to thinking surgically. They still want their shotgun. (They call the shotgun "reach," or use terms like "GRPs" to attempt to quantify reach). That's why broad, sloppy metrics like total video views, or total pageviews, or total clicks, still appeal to advertisers. They're still working from the mental model that you need to fire a big spray of shot, and that you apply some sort of fractional discount to the total number of people you fired at, and that fraction is who you managed to get. It's a dumb, top-down, unsophisticated approach to advertising -- but it persists, and so long as it does, there will remain a healthy market for "viral" services.