Because it’s impossible to prove that the phone did receive an impact. If you unbox the iPhone in front of them in the Apple Store, spill some water on it by mistake and the phone stops working, I’m sure they’ll replace it.
And the same goes for the Apple Watch. Apple's enire advertising campaign for the watch shows people swiming and underwater with the watch. But liquid damage to the Apple watch is not covered under warranty and I know someone who's watch stopped working after swimming and Apple wouldn't replace it. Makes sense....