Part of your confusion comes in the combination of artistic embellishment we take towards taking hits and forgetting what the srd actually says about hit points.
"Hit points mean two things in the game world: the ability to take physical punishment and keep going, and the ability to turn a serious blow into a less serious one."
When a character takes damage it usually doesn't mean they get cut/stabbed/rent but could keep going. Instead it's supposed to mean that they tired themselves out resisting the hit, or parrying, or moved it to another area to be a glancing blow etc. When a character hits 0, that means that in addition to all the minor wounds, they are just simply physically exhausted, to the point that 1 more movement is likely to knock you out and tear open your wounds. The body condition of a 0 hp fighter at level 1 and a 0 hp fighter at lvl 20 are about the same; the level 20 one was just much much better at knowing who to shrug off the hits or better spreading it around.
A character below 0 isn't experiencing shock, or fatal wounds. There's no arrow through the lung or guts hanging out, that's because a character above -10 always has a 10% chance of naturally stabilizing via clotting. That means that a dc 15 heal check isn't anything more than emergency first aid to stop the one or two worst areas of trauma.
-10 is not death by modern standards, but where death and might as well be dead are the same thing. Massive blood loss, shock, sepsis, internal hemorrhaging are all things beyond natural healing and the medical technology of the setting. The count to -10 isn't bleeding out, it's bleeding beyond the point of recovery because we don't have blood bags. Magic isn't going to pull your guts back in, restore blood loss, knit your crushed chest back together, postive energy mostly just refreshes and revitalizes when using the base hp interpretation.
I'm not saying that you can't tell the story where the fighter is gutted by the dragons claws only to be rendered whole by the cleric's heal or imagine it that way, just that the rules were written with something more mundane in mind.