Because backgrounds (or movies) like the ones shown in the video make the terminal text completely unreadable and reading the text on a terminal is kind of the whole point of it.

Edit: That sounded a little more negative than I meant it. I'm actually a fan of transparent or backgrounds, but I think they need a lot of tint/blur to them or they become distracting and/or make the text unreadable. This was evident in the video where I couldn't read a lot of the grey text once the background picts were enabled.

Though the use case for video still eludes me. :-)