Why are crop dusting spray booms not integrated into the wing?
Different chemicals use different nozzles and delivery systems.  Many of the spray agents are very corrosive.  Turbulence by the sprayer can be a good thing, as it aids in mixing.

Additionally drag is not a big issue as crop dusters fly slow, and tend to have good power to weight ratios, so the drag of a spray assembly at 70 knots or 60 knots, is not a big concern.  Mixing (with air of) chemicals, reduction of airframe corrosion, and ease of removal are larger concerns.