They do but chances are it's not 100% effective.
https://www.epa.gov/mosquitocontrol/aircraft-disinsection
Not sure where the idea came from that spraying the cabin aisle would penetrate the luggage bins, cargo hold, under the seats, or really anywhere.
2. Risk is convex, so every risk increase by measures not taken is multiplicative, not additive.
The real question must be: does the disinfection do more harm than it provides reduction of risk.
Wait. Is this true? f(x) = x^2 is convex but f(x+d) = f(x) + O(x). Are you saying that human risk assessment is estimated to be exponential?