User blog comment:Brothertim/Why does IW not use bullet drop over long ranges, or have delay?/@comment-1808654-20100129152527

Ballistic calculations are difficult, and so if bullet drop was included the game it would have to make a complicated calculation every single time a bullet was fired. It's likely this would impact (perhaps significantly impact) processor time for each shot fired. Whereas now collision detection is the only thing to be calculated. Might be lazy programming, might be simplifying to keep it smooth.