Every year there are examples of bullets fired in the air for major holidays and killing people after going through their ceiling. I'll take actual statistics over Google.

Unless you aim the gun at a perfect 90 degree angle, the projectile will arc and preserve at least a proportion of its forward momentum. I don't care how good of a shot you are, the ability to calculate the thin range of angles within which the projectile would no longer be able to do bodily harm, then fire within said range reliably strains credulity, especially considering velocity varies considerably depending on gun type, ammo type, and the specific load of each round (which can vary widely). So unless you have a computer in your head, a protractor on your arm, and the world's most consistent ammunition you have already tested via chronometer, don't shoot in the air.

Why is this so difficult for people?