Why is water evaporation not more commonly used to cool buildings
"This question seems rhetorically posed: how can you justify calling your solution better and cheaper without having run the numbers? – Aesin 1 hour ago"

A central air conditioning system for a house takes about 3500 watts.

http://michaelbluejay.com/electricity/howmuch.html

1 watt = 1 J/sec so that's about 1,260,0000 J/hour

A gallon of water weighs about 3.7854118 Kg

so at 2260 kJ per kg you can extract 6,780,000 J of energy per gallon of water which is more energy than the air conditioning system even uses and the air conditioning system doesn't extract the amount of heat energy from a building than the energy it uses.

If you run the numbers even with a gallon of water the energy efficiency of water usage is much superior than that of an air conditioning system.

The high heat of vaporization of water is how sweat keeps you cool. Ancient civilizations used to pour water on their tents or tepees or whatever to keep themselves cool. It's cheap and efficient. The superiority of this method when opposed to air conditioning, in dry areas at least, is very obvious. I have yet to see one good reason why it's not used more often.