EzekielRaiden
Follower of the Way
Indeed. Same reason as why HP restores fully with a night's rest, why Fighters can get physically-unexplained bursts of healing or adrenaline allowing them to act (but not move) at double speed for six seconds, why you need to be a Rogue in order to sprint and attack, why even being dropped to 0 HP leaves no lingering injuries, why equipment isn't damaged even when the person wearing it clearly is, etc., etc.Because we’re playing a game. And variable damage is interesting and fun.
The designers thought it was more interesting to do it this way--to play it this way. Because these are rules, and that shapes the experience of play. A few of those, they've built in alternative takes, but several they simply haven't.
Fiction is important. Critical, even. It is what separates D&D from something like craps. But the notion that the rules should get stuffed, rather than that the fiction should follow from the mechanical result, is definitely not how Gygax and Arneson designed the game. That is simply a fact, and things like Gygax's commentary on hit points and horses are the encapsulated demonstration thereof.
Realism is a tool. It should be used. It's far from the only tool--and treating it as though its input is always the most important, always the end-all, be-all of gameplay, is a fool's errand in D&D, of any edition.
Form follows function. And what else could "function" be, in a game, but its rules?