Why can't I see horizontal scroll bars in Finder?
You're not missing anything, this is just how Apple chose to design this view, presumably because there could be hundreds of items of each file type, but they wanted to show as many file types as possible at once. You have to scroll through using the arrow keys, by two-finger dragging, or using Shift-scrollwheel (I assume&mdash;I don't have a mouse to hand). Or you can click "Show All" to show all the items arranged in a more conventional grid.

The "stacking" of folders at the right hand side is meant to be the visual cue that this is the case.