A surprising number of conflicts: WW1, WW2, Korea, Vietnam, degenerated into artillery slugfests. You can't get much cheaper, faster, or more effective than guns shooting dumb explosives 20km away.
Only when the opponents were stupidly overmatched by US Air Superiority (Afghanistan / Iraq) did things change. But even with Air Superiority over say, Vietnam, USA still degenerated into an artillery slugfest.
-------
Its hard to beat 100lb shrapnel bombs delivered 20km away. Logistics wise, trucks carrying shells is just far cheaper than any other delivery mechanism. This fact has been true since WW1, and no technology has ever really changed the calculus. (Adding li-ion batteries, cameras, remote-control and other features so that you can turn the delivery mechanism into a drone is just unnecessary chips in most cases)