It is common to route high-speed signals on the top and bottom layers to avoid vias that cause impedance mismatches even when back drilled ($$$). To maintain a

It is common to route high-speed signals on the top and bottom layers to avoid vias that cause impedance mismatches even when back drilled ($$$). To maintain a specific characteristic impedance, you need a plane (GND) some distance from the traces which themselves have a specific width. Without a plane under/above the signals, you can't get a specific impedance value. You can additionally fill the top and bottom layers, which marginally affects the impedance.