Look at cartoon characters. No matter how many times you drop a cartoon off a cliff, or smash it with an anvil, or throw it through a wood chipper, cartoons don't die (certain film-dissolving compounds notwithstanding, you sly weasels). How do you even know that Pokemon can die? Have you ever witnessed it happening, or have you only heard of it happening through legends and stories?
I'm not saying you should never trust Alder, or the people mourning in the tower, but just as an example, everyone always thought Pokemon laid eggs even though nobody (even the Daycare owners) saw it, up until the Sinjoh Ruins event showed us how Pokemon Eggs are really created. How do we know Pokemon "death" isn't the same concept?