For the same reason we should use Sky+ or Tivo. We've all got to move on. It's far too easy to keep cranking out shit that interrupts people while they're trying to do something else. It time to stop deluding ourselves that by doing so we're being creative.
Yes, yes, I know. Banners and buttons and tv ads are all part of the media mix. There's a time and place for everything. Turn, turn, turn. And there's truth in all that. And the pencils for tv ads and web banners look just like the ones the guys at R/GA won for Nike+. And I do wish I had "Sony Balls" on my reel.
But Jesus H. Christ in a chicken basket, how much more fun is it to make something useful? Something enthralling? Something that people choose to engage with, rather than something they merely tolerate? Maybe if our banners were a "front door" to something more interesting, or if our TV ads played a part in something bigger - maybe then we'd be on to something. But admit it. Most of yours aren't doing that. They're just irritating little rectangles or stretches of time whose highest aim is to amuse someone just long enough to pick their pocket.