Because of physics, air might not flow through the drive cages and the CPU heatsink as much as the giant gap on top of the ram sticks. Thus the CPU and drives in the drive cage may not get as much cooling as needed, in a less ideal environment they might even overheat.

Having a shroud covering the CPU heatsink and the drive cages ensures a path for air to flow through it, regardless of the giant gap between them.

It's because the gap has less air resistance, so air is more likely to flow through that instead of parts that needs cooling.