A warning shot is a shot that is intentionally misfired in order to scare potential predators and give them sufficient time to flee. Allowing warning shots to be legal likely reduces the amount of aimed shots for which the intent is to cause harm. Fewer aimed shots means fewer deaths and injuries due to firearms as well as fewer arrests for unlawfully harming others.
A. The primary reason is that you can not be sure where the bullet will land. Physics 101, Anything that goes up has to come down. And any responsible gun owner must not gamble with unknown people's life's.
B. Read news from different countries and you see how often "lost" bullets fired in parades, or by police themselves find a couple of kids a year. Physics isn't a fairy tale.
C. To be brutally honest a warning shot might trigger an unpredictable response in the aggressor, it could be fight or flight, you cannot be sure. Then you are faced with a fight after missing your chance of ensuring the aggressor isn't capable of injuring anymore.
D. Make your first shot count period.
E. Last but not least...Do the rest of us a favor and save us the tax payer money.