And that's one possible narrative. It postulates one kind of fiction for D&D's world - fireballs are basically instantaneous explosions. I'm asking why D&D is using that narrative instead of the, "Spells can be dodged by the nimble" one, where a fireball moves that much more slowly.
...and? You're rolling a saving throw, no? Based on Dexterity? If you save, you should "save" yourself from the whole effect, just like with Hold Person. Kind of like if you "miss" with a sword, you should really, really miss, right?
My point is just this: These are arbitrary distinctions.