The Recent Apps list is not a task manager, so there isn't a Close All button. Actually closing all apps isn't very helpful for the system, because it will just open a bunch of apps again to full up RAM (since that's how Android manages RAM). If you did that frequently, you'd end up using more battery.
I would agree, though, that it would be nice to be able to reset the Recent Apps list with one touch, just to make it less cluttered.