The thing about Upward is that it's a lot less open than it looks. Let's look at it from BLU's perspective. To your left, your view is mostly blocked by all the buildings in the middle. On your right, a sheer cliff face with almost nothing but empty open space beyond it. So most of what's rendering is up ahead. The map is essentially a hallway that spirals around.
There are a lot of hint brushes, and as usual I don't really know what most of them do, but there are a few important ones: the ones that stretch across the entire map and split it up into invisible, horizontal "floors". These prevent the game from rendering what's on the other side of the high walls. Here's an illustration:
The solid lines are the walls, and the dotted lines are the edges of the hint brushes. You can draw a straight line from that pink zone up into all of the white slices, but that's OK because it's up in the air and there's hardly anything to see there. But in order to see from pink to yellow, or green, or blue, you'd have to draw a line that curves or bends back down again. The game knows that seeing doesn't work that way, so it doesn't render anything in the other colored squares.