I consider it lazy and devoid of imagination to assume that there must be a rule covering everything that could possibly happen in the game world.
That's fine. I'd say it's foolish, myself, but the point is that neither of us want such a thing.
Something are not worth including in the simulation provided by the rules, because it is not desirable for them to happen by random chance. Some things are not worth including in the rules because they are so rare and freakish that it's simply not worth the time to model them and they'll only happen if the DM wants them to happen.
In both cases, such things happening in the game reek strongly of DM Fiat. And if there's no way the PC's can interact with such a thing (using the rules), then it really is largley equivalent in feeling to saying "Rocks fall, everyone dies." The DM just took a big ol' broom and swept away any feeling like I, as a player, have any true influence over the world. After all, if the DM decides, I could just fall off a horse and die. So if it's all in the DM's hand, why bother having rules at all? Why bother having a game? The DM can just sit and tell us his story, because he gets to dictate what happens without rules anyway.
I mean, that's hyperbolic, but it's the feeling I get when the DM breaks with the rules of the game so dramatically just to justify some sort of narratrive contrivance that, 9 times out of 10, a little imagination could have worked within the bounds of the rules to create something the players COULD interact with, and thus could have added to the game, rather than made me feel like I was just along for the DM's ride.
Nobody wants to roll a 1-in-a-million chance for a high level fighter to fall off a horse and break their neck whenever they go for a ride. It's a waste of time, and not desirable as a random outcome anyway. That does not mean that it's impossible for a high level fighter to fall off a horse and break their neck, entirely bypassing the hitpoint simulation.
Sure it does. I don't know of once in all the tales of epic heroes where someone fell off their horse and died. That's not heroic fantasy at all. That's the cold, hard, jagged stone of unnecessary realism cutting to ribbons my little fantasy world where heroes make flying leaps from falling dragons and land on the backs of their trusty steeds.
A one-in-a-million chance doesn't, effectively, from the POV of the table, ever, really, truly exist. And if the DM calls it in, it blows my suspension of disbelief right out of the water, because no longer does my character adhere to the heroic archetype I thought she was. Now, she's as vulnerable as a peasant just, out of the kindness of the DM's heart, lucky.
That's immensely unsatisfying for me.