In short, both a TV program and a magazine represent a finite unit of (more or less) undivided attention. Each of these media objects is carefully designed to grab your attention and to hold it within a bounded space—with boundaries being the start and end times of the program for TV, or the two covers of a magazine.
What advertisers buy when they purchase a magazine or TV ad is slice of the attention of some subset of that media object's audience. And the ads that they create for those purchased slices are attention-worthy objects in and of themselves, e.g., Angelina Jolie posing with a diamond watch, or a hilarious vignette centered around a brand of beer.
A web page, in contrast, is typically festooned with hyperlinked visual objects that fall all over themselves in competing to take you elsewhere immediately once you're done consuming whatever it is that you came to that page for. So the page itself is just one very small slice of an unbounded media experience in which a nearly infinite number of media objects are scrambling for a vanishingly small sliver of your attention.