My educated guess:

This addition would likely require regular cleaning so contamination (dust, pollen, etc.) doesn't accumulate in the radiator cooling fins.

Plus the evaporation rate depends:

• on the RH of the air.
  
• exposure to sunlight
  
• Air temp.
  
• Air flow

So you'd want a way of controlling the rate of water flow; adjusting for these floating variables.

Also, the water would generally need to be drawn through the sides, and blown up/out. This isn't an difficult obstacle, but it would change the design.

Plus, the liability of water damage from:

• poor installation
• leakage
• neighbor kids screwing with it
• Damage/wear from increased thermal cycling (cold water of hot surfaces) literally rinse and repeat.
  
• etc.
  

Plus the warranty risks associated. Adding water to anything creates problems.

Also, it reduces profitability for the energy companies.

It's a good idea. On hot days you could momentarily boost you efficiency by spraying down you external ac unit, but it will evaporate quickly.

If your ac fins are bent then you could buy one of these to straighten bent fins and increase the air flow.

You could also winterize your windows to reduce leakage and basically add an extra insulating later to your windows.

This is all shooting from the hip guesses.

I'm not an AC engineer...

Good luck!

Edit:.

https://en.wikipedia.org/wiki/Evaporative_cooler

Evaporative cooling is especially well suited for climates where the air is hot and humidity is low.