If magical beings who can conjure matter, teleport, and manipulate the physical universe can be picked off as easily as "Hur hur, drop bombs on them--WIN!" then how the hell did the Axis lose against regular humans?
Wizards stomp. They're not some mobilized tank division, infantry battalion or a city. They're individuals who can literally vanish in an instant and practically hoodwink the entire muggle world in to making them think they don't even exist. What's an artillery spotter gonna do?