[frob]

There are way way way more of them, so if you randomly select from the set of all possible configurations, you are much more likely to get something spread out and chaotic then something with a recognizable structure. As atoms and molecules bounce around, they are effectively randomizing. After a few dozen interactions each, they're basically in a new state. Repeat over and over and you're basically just drawing from that same set again. There's a chance that all of the air molecules could bounce to one side of the room at once, but the likely hood of that makes it something you won't see in the lifetime of the universe. So if you see a video where all the air rushes from half the room to fill the rest, you can be effectively certain you're watching the video forward and not in reverse.