Edit: No really, I'm just asking. Why can't they? Is it banned? I honestly remember hearing the hoopla around background applications a few months ago.
The iPhone does enable Notifications, which allow you to receive an aim message, for instance, while you are not running the AIM application, but this is more akin to having AIM send a special version of a text message to your phone, asking you to open AIM to see the full message.
There are ways to use background applications on Jailbroken iPhones, but the UI isn't designed for it, and they run slowly on anything less than a 3gs.