Air strikes are far more accurate - if you think of the 'flight' of the bomb as being from the time it leaves the ground to the time it comes back down again, an air-dropped missile is being directly guided and still possible to call off for the overwhelming majority of its flight, and then guided by computers for the rest of it. This means you can normally be sure that you're hitting what you think you're hitting (OK, you can't distinguish friendly soldiers from enemy soldiers easily, but you can at least tell that you're hitting soldiers rather than a forest or something), and that the missile will actually land where you want it to - it's never just being left to fall. By contrast, an artillery shell is being fired from tens of miles away, by somebody who can't actually see the target, and once it's gone, it's gone - the only way to make corrections is to note where the first shell lands and adjust the aim of the second shell. In fact, it's quite unusual for even mortar fire to be directed spot-on with the first round in a combat situation, because that depends on perfection both from the infantryman on the ground and the mortar team - it probably says something that I still remember just that happening once.
