I was going to call this a misconception. But, in fact, the answer is that Twitter is a high-bandwidth medium. For readers.

(It's more effort for writers, but who cares about us? There are orders of magnitude more readers than writers.)

It's true that I could dump a lot more raw information on you with a ten-second video Tweet than a 140-character text Tweet. But most of that information would not only be useless (you probably don't care to know the exact quality of the light falling on the bookshelf behind me, or how well I shaved this morning) but positively negative (because it will distract you, detract from my point, and tend to engage and distract many more levels of your brain, thus contributing to the mental cost of context-switching that is the bane of modern existence).

What I crave as a reader, among other things [1], is a source of plain-text links and messages (which can be followed up if desired) that are drawn from a body of people I vaguely trust, or their immediate circle of friends; that have been wordsmithed, at great effort, to pack the maximum content in the minimum space; and that have been arranged on a single screen that can be browsed without fuss or effort and that doesn't contain big blinky ads that have been engineered to be among the most distracting things ever made. And, what do you know, Twitter is a pretty good approximation of that, especially for events that are evolving in real time.

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[1] This is kind of an important point. Twitter is one tool, for one kind of reading. I wouldn't entirely give up blogs, or YouTube, or HN for Twitter, just as I wouldn't give up all books for Wikipedia.