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LAUNCHXL-CC1350: Why are the signal/power tracks routed an internal layer?

Part Number: LAUNCHXL-CC1350
Other Parts Discussed in Thread: CC1350

I was looking at the design files for the CC1350 launchpad and I was curious about something I noticed: All the signal/power tracks are routed on an internal layer, leaving the bottom layer blank except for a ground plane.

Given that the layer below the top layer is a ground plane anyway, I didn't imagine it contributes to the RF performance. So why is the layout like this?