The problem is that air gaps not only influence the base capacitance, they also influence the capacitance of the touch sensor when someone actually touches it.
The air gap essentially acts like another capacitor that you put in series with the touch sensor. This additional series capacitor worsens the coupling between the microcontroller (or other sensing circuit) and the actual sensor.
Here's an example: Let's say you have a touch sensor that has 10pF capacitance when it's idle, and 100pF when someone touches it. That's a difference of 10:1, and easily detectable.
Now let's add an air gap with an equivalent series capacitance of 10pF. This means that, in the idle state, you have a 10pF air gap in series with the 10pF of the sensor. That's 5pF when the sensor is idle. When someone touches the sensor, you still have the 10pF air gap, but in series with 100pF from the sensor. 10pF in series with 100pF is about 9pF. Now the difference between the "idle" and "touched" states is only 1.8:1! (9pF when touched, 5pF when idle.) This is much, much harder to detect.
So, in this example, adding an air gap reduced the sensor's sensitivity by more than a factor of 5.