It doesn't. However, where does it say never do it? Furthermore, I believe I've seen examples of such dialogue in earlier editions, but I'm at work and can't quote anything.
When a player character can heal someone up to full hit points with a few words of encouragement, the only way to avoid the situation of describing a gruesome wound and then having them heal it in a way that defies suspension of disbelief is to never describe such a wound. You must talk around it. I think it's fun to describe combat, and sometimes I like to describe a nasty wound. Sometimes losing hit points, especially a lot of hit points, is a wound, not fatigue, or narrowly dodging something, or luck running out, but a gushing wound. But non-magical healing makes that problematic. The mechanic works against the narrative that I sometimes choose, and I don't like that. I could choose another narrative, but I feel that I shouldn't have to in this case.
Describing a gruesome wound using hit points defies suspension of disbelief.
Quick anecdote. I've had a pretty bad year and a half (extraordinarily bad for most people but this is just pretty bad for me) with sports related injuries. These put me on the shelf for certain activities in some way, shape, or form for 14 out of those 18 months. In order:
1) 5th metacarpal fracture with partial ligament tear of the right hand. 6 weeks, I pushed the recovery time, reinjured, then 4 more weeks.
2) Traumatic left patellar (kneecap) bone bruise. Brutal injury but only 3 weeks.
3) High grade 2 (more than partial tear - could have had surgery) left ankle sprain. 4.5 weeks. I again pushed the recovery time but both of my ankles are toast so it doesn't really matter at this point.
4) Disgusting dislocation of my index finger on my right hand. Horrible volar plate injury that will never, ever, ever be right again. The rest of my life I'm going to deal with terrible pain and some lack of grip strength, dexterity, and flexion with that finger/knuckle infrastructure. This has been a hellish injury that was ongoing for 6 months before I could do much of anything with that hand. 9 months later and I'm using it with a resignation of its diminished capacity (and terrible pain). This is the infamous "turf toe" injury (but on my hand) that has ended many a football players' career (Deion Sanders being the prime...time...example).
5) Horrible Illiac Crest (vertically ascending part of hip) bone bruise with a crushing damage to the soft tissue across the front of it. The infamous "hip pointer." This was only 2 weeks (I pushed it) but it was worse than number 2 above.
6) Weird small shoulder fracture on the stem of my left acromion bone right where it meets the clavicle. 5 weeks.
Any sort of "gruesome wound" is not remotely modeled by HPs. Not even in the same universe. Certainly none of the above would be modeled by hit points and these are probably relatively minor compared to what you have in mind. The actor's capacity isn't even remotely inhibited let alone outright nullified by any HP loss except the loss of the last one. A death spiral mechanic on top of them or some kind of condition/injury track sufficiently does the job, but those aren't terribly fun. So we have hit points. And we each do with them as we may to tell our stories as we might. So narrate the resolution of your action declarations as you will but its not very tenable to tell folks that this or that mechanic ON TOP OF or INTERFACING WITH hit points reduce your ability to model "gruesome wounds." Hit points do all the work themselves to make it problematic for you. They don't need any help. Just look at it with squinted eyes, or outright avert your eyes, the same way we've always done.
